I think many kids start to think about giving up their naps at about this age. Sleep is really important for kids (it's when they grow!) but most parents struggle to help their little kids get enough sleep. When she does nap does it mess up her bed time routine? I have a three year old and if he naps during the day he won't fall asleep at night until after 9:00!! (So we actually try to keep him from napping during the day).
Have you tried just having her rest even if she won't fall asleep? When my older son started to give up his nap, we'd have him stay in his room for an hour even if he didn't fall asleep. We let him look at books or play with a special box of quiet toys but he had to stay on his bed for the whole time. Even that little break helped him have energy and a good attitude for the rest of the afternoon. (And of course it was a great break for mommy!).
Let me know if you try that and how it goes. Good luck!