My (28/F) boyfriend's (32/M) ex-girlfriend (30/F) from just over a year ago sent me a direct message with screenshots claiming that they had spent the night together when I was away on a business trip a few weeks back, but he denies it completely.

They were together for a couple of years. On and off. He and I were friends at the time. When they broke up, I noticed he had started making some good changes in his life. He was more focused on work, began to attend therapy and cut out a few of the more toxic friends that he had kept around. A few months after my ex and I had broken up, my current SO and I had decided to date. He asked me out and I figured it worth a try, so I went for it and we've been together ever since.
We don't yet live together as it's still a bit soon into the relationship. But we see each other all the time. At least five or six times a week. We both work a lot, or we would probably spend even more time together. Lately he has been looking at a few apartments closer to where I live. Things have been going well between us. We haven't had any arguments. Naturally we've disagreed on a few things here and there, but it never got to the point of arguing or having to apologize for what was said.
It's a mature relationship. I truly feel good about where we are as a couple.
But of course, last night I got a direct message from his ex. At first I didn't know what I was looking at. I didn't even know it was her until a moment later. She and I had never really hung out outside of a few group situations. Anyway she wrote a lengthy message, saying that my boyfriend had sex with her when I was away on business a few weeks ago. She said it happened after they had crossed paths at a pub one night (a night that I remember because he told me that was going out for drinks with his friends). They said hello and exchanged numbers again. After a few drinks and after everyone else had left, he walked her back to her apartment and agreed to come up when she asked.
They had sex. He left immediately after it was over and then sent her a string of text messages saying that it never should have happened. Those were included in the screenshots. He was basically like, "That was a mistake. I'm sorry. I can't deal with this right now. I have a girlfriend and I love her. I don't know how that happened but I have some **** that I need to figure out. Please don't contact me again." After that I guess she felt she had to say something, probably just to get back at him for cutting her off again.
This happened three weeks ago. When I asked why she hadn't said anything sooner, she said that she was thinking about it and had tried to put the entire night into the back of her mind but eventually realized that she couldn't.
I asked my boyfriend to stop by my apartment after work and he did, and I didn't say anything. Instead I just opened the screenshots on my phone and handed it to him without explaining what was going on. I wanted to see his expression. He seemed confused at first, as though he didn't know what he was looking at. Then he recognized his ex's name and photo and the pieces fell into place. Finally I asked, "What's going on?" and he replied with something along the lines of, "I have no idea. These aren't from me."
He did most of the talking, basically explaining that he had in fact crossed paths with his ex at the pub that night, but that nothing happened beyond an awkward greeting. When I asked why he hadn't told me sooner, he said that he didn't feel it was important enough to mention. Because nothing happened. When I asked why his ex would go so far as to send me screenshots of an exchange that had never occurred, he insisted that she was probably just trying to get between us and reminded me that she was toxic for the entirety of their relationship. She was and no one really liked her. Not his friends, not his family, not his coworkers, etc. But to say she would do this after a year of us being together ... it feels a bit ... well, it feels unlikely is all I'm saying. Wouldn't she have pulled this sooner?
He agreed that it was strange of her to wait all this time but he also said that she was probably hoping something would happen after they had run into each other. When he refused to speak to her for more than a moment, she probably felt type of way about it and then sent me bull**** screenshots to get back at him for paying her no mind.
I asked if he could prove it and he said he would call his phone company and get a transcript of some sort to prove that those messages were never sent from his number. It's been a day. When I asked if he had called the phone company yet, his response was, "Do you really think I would cheat on you?"
To me that's about as clear as it gets. The fact that he didn't contact them at his earliest convenience ... just to put this to bed ... it looks bad. Really ****ing bad.
I replied with, "I don't know what to think. That's why I need you to put my mind to ease like you said you were going to."
He agreed that he should have done so right away and said he would sort it out after work today.
Now I'm just waiting and wondering what to think. His ex sent me another message today explaining an idea for a tattoo that he has been thinking about but hasn't gotten yet. How would she know that unless he had told her on his own? That means, at best, he had lied to me about the length of their conversation at the pub. At worst he lied to me about all of it and had in fact cheated on me that night. Either way he's lying and I know that for a fact now. Nowhere on social media has he posted about his tattoo idea. He hasn't even told his friends because it has to go with their group and he wanted to surprise them.
I was the only one he had told. Until now I guess.
What should I do? This is so draining.
Update: He didn't cheat. His ex's story unraveled and he showed me his phone records to prove that the screenshots weren't real.
tl;dr He says that she just wants to break us up and insists that it's all a lie. But a few details suggest otherwise. Not sure what to think or how to feel. This is such a shock to me.


Source.