I may have accidentally sexually harassed a coworker and I need to know how to solve this. We go back to work on Monday.

I was heading to the finance department yesterday, just before ending the day, to get some clarification on something. I was walking behind a female employee who is around my age. I actually didn't recognize her but she definitely works for the company. Her phone rang, she went to answer it, apparently it was in her boob cuz she reached in there, but then dropped it as she pulled it out and she bent over to get it.
I don't know if she knew I was behind her or not but she bent over too quickly for me and her ass went straight into my crotch and on instinct I grabbed her waist/side of the butt so I didn't fall over. I wobbled to the side and smacked into the ball and we locked eyes....I froze then chuckled nervously and walked on.
Later in the evening, I randomly thought of the incident and have been paranoid that she thinks I grabbed her lower area on purpose. I swear on my life it was instinct. I need to know how to handle this. If she goes to HR on Monday, they will believe whatever she says and I will lose my career. I cannot lose my career. I worked hard to get where I am at and I cannot get this out of my head. I was thinking of showing up early and waiting for her to pull in so I can apologize. Is that the smart thing to do or is that stalker ish?
How do I handle this? If I lose my job I will lose my life.
EDIT-- thank you for the advice! Keep them coming. I emailed HR and briefly advised of the situation And requested a meeting. I also emailed my direct boss. Hopefully nothing negative comes from this.


Source.