12 week old arching back and turning on side when put down to bed...is this normal?

For the past 2 days, when I lay my 12 week old down flat in her crib of pack and play, I often find her arching her back and head and turned to the side. She doesn't seem uncomfortable, she's just sucking on her pacifier, but she's got her head turned up and her back arched and she's almost rolled on her side. Is this something to be concerned about?