Hi B.,
My 2.5 year old son was 18 months old when he started to go through the exact same thing. I looked it up online and was surprised to find that it happens to many, many toddlers! It happens out of the blue, leaving parents completely baffled! I think this may have lasted for about a month, probably a little less than that. The real problem was trying to wash his hair--he screamed like I was pouring hot oil over his head or something. I don't remember exactly how he got over it, but I do remember just trying to make bathtime seem like such a fun thing to do; oh, and I'm pretty sure there were plenty of bribes too (hey, you gotta do what you gotta do.) I also switched from giving him baths from evening to earlier daytime hours; it kind of threw him off a bit, but it seemed to work. I also would let him take showers like "mommy and daddy"; he STILL thinks showers are a fun alternative every now and then (we pretend it's raining on him.) To this day, for some reason he enjoys the water pouring on his head from the shower when he stands under it himself, as opposed to him crying when I try to rinse the shampoo out myself with cups of water. It doesn't matter if the water is constantly running in his eyes; it's almost as if it's okay because HE is the one in control.
If you haven't already, you can also try buying some of those bathtub crayons or other crazy products out there that are supposed to make bathtime more fun. Or maybe you can let her wear her bathing suit and tell her she's going swimming; kids seem to enjoy the simplest "adventures."
Good luck, and remember, this is just a phase and it will pass!!