Is she sleeping all night with no wake ups?
Has anything changed during her day? Daycare or anything?
Try to think of anything that can have happened.
Does she sleep all night regardless of her naps?
Like Stacy said it could be due to a drop in her sleep requirements, but if she doesn't have a long enough nap, then I would put her to bed at 6.30-7pm to ensure that she gets her night time rest.
I have had sleep problems with Alex lately, she is resisting bed time, but have found that if I cut her nap to 1.5-2hours then she is better.