Naptime = play with my poop and pee time

I've been laying Nina down for her nap and she is just laying there and I say "i love you" and she says "you love me" (because of that dumb barney song) anyways i'll keep the monitor on while im doing something and hear her awake. Well ive figured out that if shes up too long shes up to no good. I walk in and she goes "oh no...oh no, ook mama ook" and she either smeared her poop or peed somewhere on her bed. Does anyone know how to remedy this? i try going in sooner when i hear her up but i dont even know when she starts doing this mischieve. Anyways, it's pretty hilarious but i want to stop having to throw everything in the wash after nap time.