Okay mama's. Sleep question for my 5 month old. I always put her down on her back to sleep, but…

Okay mama's. Sleep question for my 5 month old. I always put her down on her back to sleep, but lately she is rolling onto her belly. She knows how to roll back to her back but she prefers to be on her belly. I get super nervous about it and flip her back to her back. Is it okay now that she is closer to 6 months to let her stay on her belly. There are no bumpers, pillows, blankets or stuffed animals in her crib.