shell shoftness?

Hi again I have a northern and I just noticed last night, the little guy was basking up against the glass (like standing I guess) and he usually falls back and flipps himself over. then I notice when he was crawling away that his top shell right above his tail was kind of curving down. So I picked him up and I could bend it abit back alittle. Is that normal for a turtle to be able to bend like that? I think it started to curve when he stand up like that against the glass. Today its alittle bit better. But do I have to drydock him for awhile or something? Paul