Yeah, that could definitely be it. A lot of herps have real trouble adjusting to colder climates, especially animals like that. I had an Iguana that had trouble in Chicago as well.
I only took photos of one of them, but here you go:
It just sucks cause I would love to have a turtle sanctuary (there are sooooo many neglected turtles out there) but alas, I can only handle so many...
Bart