Have you inspected her paws? Can she walk at all? Is there any obvious injuries?

I would take her to the vet and get her checked out!

Please explain better.
Is this a dog lying on her back when she just doesn't want to go somewhere or do something, a typical response of submission/refusal, or a dog doing this at all sorts of times around the house and outside, for no apparent reason?

Maybe you need to tell us additional info. She may do this because she trusts you trying to show that she respects you by revealing her belly. Because the belly can be easily torn open by another dog this is a sign of respect.