My girlfriend is acting weird (or at least I think so)

My girlfriend and I are dating for about 6 months already, and we're both studying time consuming careers (she medicine, and I engineering)

So from the beginning we knew we would have little to no time for each other...

The thing is, up until today I was very busy with some exams and we didn't see each other for something like a week. We were planning to hang out today, but I forgot that I had a more important appointment at the only time we could see each other. I told her this morning and she seemed upset, which made me feel really bad (and I know I deserve it).

Now she doesn't want to see me tomorrow (in my birthday) because she says she doesn't feel like it anymore...

Is she being childish or am I just making it up?

If she's being childish I'll ignore her for now and we'll talk about this when we see each other, if she's not I don't know what to say other than sorry and buying her some chocolates...

Edit: corrected an excessive use of "today"

Which one do you think?