I would draw her aside and tell her that you are sorry for forgetting her birthday, but that her behavior makes you worry that something else is going on with her. I wouldn't get her anything now -- it's too late, IMO. Anyway, she's psycho if she can hold a grudge this big for this long over such a small thing.
Then I'd wait a couple of days and give her a list of supplies you need. If she's not going to get over it, that's her problem -- she still needs to do her job.