Hmm. This is one of the reasons I've decided to not have roommates. I'd probably go passive aggressive and leave little post-its with my name and things like "do you really want to eat MY food?" written on them.
I definitely agree with the consensus to talk to her when you are calm. Perhaps write down the points you want to get across in case she tries do derail the conversation. It may help you from getting your temper up as well.
If things don't change, and you can't move out right away, maybe think about getting a mini-fridge with lock for your room to keep your favorites safe.