T.H.
How much is she eating? Could that be it? I found that the best way to get my kids on a good sleep schedule was to make sure they were eating on a schedule. I did bottle feed so that did help and it would be trickier while breastfeeding, but still doable I think. I should add that I fed them every three hours and if they were asleep I woke them up. Both my kids were sleeping through the night at 2.5 months for my daugher and 4 months for my son, and by sleeping through the night, it was a solid 8 hours at least.
Is she over stimulated? Do you have lots of outings or visitors or lots of noise going on all day? Maybe she's just over it and can't calm down enough to sleep.
The first six months is the hardest! Hang in there and I hope you figure it out!