Baby forgot how to roll over?

Baby girl is 6 months old today and rolled onto her back at 3 months, but ever since she rolled from back to belly at 4.5 months, she hasn't really rolled back the other way.

Last night she rolled onto her belly in the crib for the first time and started crying.

It's hard to tell if she's upset because she can't roll onto her back, or because she wants to crawl. I keep trying to get her to roll on her back at tummy time but it's almost like she forgot.

She goes up on knees but still only pushes up to elbows and hasn't rocked yet.

Has anyone else experienced this where it seems like they forgot how to roll the other way?