C.F.
One of the issues may be the feeling of the bottoms when she is napping, she may like the feeling of the sheets or blankets vs her bottms. Could it possible be that she feels movement restraint when the bottoms are on or at the very least could she be to warm? Another option to look at is the timing of the nap i.e. is it too early or too late? Nap time sometimes needs to be adjusted as they go through different stages. Children start developing there likes and dislikes very early and need to be given there independence to explore them. Compromise and use a night gown or add extra blankets.