No surprise at all! You should talk to her and get her to start enjoying anal when she's sober too, and then work to convert her to anal only. After all, if she squirts and cums so much better from anal, she should be doing it full time.
I have a fuck buddy who doesn't like anal unless she is drunk. So I get her drunk, and she squirts normally pretty hard. But when I am in her ass it's a constant waterfall. Good thing I have 2 beds, cause if I'm in her ass, no amount of towels can keep the bed dry.