You mean when you are holding her? Or when she is laying in her crib she bends it back?
I am sure it is fine. If she were uncomfortable, she'd move it. If you're holding her and she does it, just move your arm to bring her head forward. If she does it in her crib, I wouldn't worry. My son does this sometimes too sleeping in the crib- he cranes his neck back really far. He always ends up bringing it back to a more comfortable looking position later, though. But definitely talk to your dr. about it. I don't know of any medical reasons for it, but there may be a reason why your baby does it