Thanks for your reply.
It kind of looks like it could be a game she's playing. She literally scoots along under her food dish on her back, wriggling her feet in the air then jumps up and bounces around. I tried getting her attention when she was doing this and she was immediately over to me, sniffing to see if I had treats. Same with the arching her back and neck and looking up, she can be distracted very easily and doesn't seem dazed or in any way off. I suppose I'm just concerned because of the incident that there may be some lasting damage to her neck. The other two don't do this and I've never seen her do this before about a week ago. I've gently checked her neck and there's no lumps or strange bits and she doesn't seem to flinch or be uncomfortable.
If she was in some sort of pain or discomfort, she'd probably lose her appetite or her poos would be abnormal, right? Am I worrying for nothing when she's probably just found a fun new game?!