Sounds like shes stretching to me ,how old is she, is she too young to not go in the High chair, are there any other issues with her, If you are concerned about her health go and get her checked out.Maybe the high chair is too high and shed rather be lower, some high chairs can be fixed to be nearer to the floor.
Chloe is 9 months old. She also falls asleep someimes in this arched position. She also will rock her pelvis like shes uncomfortable. Again, she is quiet when doing this and looking up at the ceiling. My son tried to wake her and get her attention, but no response. I picked her up and she fell asleep. My son said that it didn't look right.