Another thing to look at is to check on her frequently in the last half hour of naptime. There is a chance given her age that she is not sleeping the entire time. It doesn't matter whether I sleep 3 hours, 5 hours or 8 hours I need to pee as soon as I wake up. If she is awake and just laying there she may be doing her best but can't hold it till the entire rest time is over. In which case a pullup will prevent the mess but you might also need to address whether a shorter time is necessary. Not all kids sleep two hours as in let her get up after the 1 1/2 hours and read a book or something after going potty.