where did my sweet baby go?

At some point in the last week and a half, my sweet, happy-faced, good-natured, adorable little girl was replaced in the night by a two-foot tall screaming harpie. I'm not sure exactly when it happened, but I'd like to know how to change her back.

Partly, I think, it's because as my pregnancy progresses, my milk supply is dwindling** and she's frustrated that she's not getting as much as she wants, and I'm guessing that it's also a developmental thing. I'm hoping that it's just a phase - she has the most ear-piercing screech, and she's so bullheaded (umm, I guess I'm to blame for that) that she'll scream for half an hour if thwarted over almost anything - being picked up when she wants to be down, being put down when she wants to be up, being removed from the kitchen (she's not allowed in there at all - that's where the cat eats and the recycling lives) not being allowed to flush a roll of toilet paper, etc.

I don't think she's old enough yet to understand the concept of "time out", and she's too single-minded for ignoring her to work as a tactic - she'll just continue screaming. How do you deal with a 15 month old when she starts throwing temper tantrums?

**(A resource that Cheryl suggested, and that I'm finding really useful is the book Adventures in Tandem Nursing. It's a must-have if you're even considering nursing through pregnancy or nursing two babies.)