If its molars coming in, then there's a chance that she's a bit OT by nap time. Teeth can knock the stuffing out of LOs and make them more tired than usual. You might try a slightly earlier nap (15-20 minutes) and see if that makes a difference. If that doesn't work, bring it forward another 15 minutes (total of 30) and see what happens.
Of course, OT and UT often look the same!
If the 30 minutes doesn't make things better, than go the opposite direction by 15 minutes and see how that works! But my *best* guess is that the molars are making her more tired than usual. My son is a great sleeper like yours, and those 2 year molars gave us 15 hour sleeps for awhile!
We moved his nap and bedtime earlier by 30 minutes each, and he started having 13 hour nights.
Sadly, those days are over. But maybe you'll see something similar!