Girlfriend (24f) keeps eating food I (27m) bought for myself and gets defensive when I mention it

My girlfriend and I split the cost of grocery shopping 50/50 since pretty much everything we buy is for both of us anyway. Neither of us really eat breakfast but recently I ended up unexpectedly losing a lot of weight so I have started eating breakfast.
Since the food I bought is just for me, I paid for it myself and got it separate from the grocery shop. I bought myself a range of things so it would last and so I wasn't having the same things each day and so it would last me a while. Once my girlfriend noticed what I bought, the next days she started using it to make herself breakfast.
I asked if she wouldn't use much since I've bought it for myself since I need to gain weight and that it was quite expensive. She said she wouldn't use much but then the next day she did the same thing. I mentioned to her that since she's now eating breakfast, we'll add the food to the grocery shop then.
She said no since she is only really having it because we've got things in, she doesn't really need to have it. I pointed out if she's going to continuously use the food I bought for myself then the least she can do is contribute towards it. She said she doesn't want to pay for things she doesn't need but I just pointed out she's more than happy to keep using it once someone else has paid.
I just said if she doesn't want to pay towards it then I don't want her having any more of it since I'm having to buy it more regularly if she's going to keep eating it. She said I was being selfish and unfair but I just pointed out she just expects to be given the food for free.
I said I'm happy to share it as long as she starts paying towards it but she just accused me of making it all about money. I just repeated that if she's not paying for any of the food then I don't want her having any of it. I don't think I'm being unreasonable here, does anyone have any advice on how to handle this?
tl;dr I bought myself food for breakfast since I need to gain weight. My girlfriend has started eating it but refuses to actually pay towards it. When I brought it up she got annoyed and told me I was making everything about money but I just told her I don't want her eating the food if she's refusing to pay towards it


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