My son was the same at that age. I think it was a combination of getting older and of being able to do more before becoming tired. His usual 2 1/2 hour nap (1-3:30) went down to 1 1/2 hours (4-5:30). I started taking him to the park more to make sure he was getting enough physical activity, and that lenghthened his naps a bit, but other than that, he just seemed to be growing out of the "long nap" phase. He continued to take afternoon naps till he was about 4 years old, though.
If after a few weeks she's still not sleeping, I'd talk to your doctor, sometimes lack of sleep will actually cause kids not to nap (which seems weird, since you'd think they'd nap MORE, but sleep deprivation makes the body act weird). Every child is different, but I agree, I think 2 years old is too young to start staying up all day.