This has so happened to my DD!! She rolled from tummy to back first when she was about 5 1/2mnths old and then rolled from back to tummy when about 6 mnths old. Now she rolls from back to tummy all the time and does 360's but then cries because she can't remember to how to roll back!! My health nurse said it's normal for them to forget things and just to remind them 0 but don't stress. It's like once they learn a new skill they forget an old one for awhile!