I would not worry too much about it if she is able to roll over on her own. All of my children have always prefferred sleeping on their bellies, and they are all happy, healthy children. For most babies, it makes them feel more secure, like they are snuggling up on someone.
I do realize we are currently taught to have them sleep on their backs, but 20 years ago, THAT was the horrible thing to do (they could spit up and choke) now, we are told they could suffocate if on their bellies (I reassure you, not once have I ever heard of this being the REAL reason a baby died) Who knows, maybe anothe 10 years and it will be back to putting them on their bellies.
I think it is more important to reduce the amount of blankets, pillows, stuffed animals and other items in their crib, and to let them sleep however they are content to sleep. Dress them in warm pajamas if it is cool in their room. good luck!