Rolling over?

This morning I put Kate on her tummy to play like she normally starts off the day and walked into the kitchen.  When I came back, she was on her back looking up at me.  I'm wondering if it was a fluke, or if she is starting to learn how to roll over.  She has rolled from tummy to back a few times in the past, but she was upset and arching her back so I never counted that. But this morning she wasn't upset, until I tried to put her back on her tummy to see if she would/could roll over again.