They can be like us ladies we had a mixed breed female who only came in to heat evey 3rd year so after she became 8 we figured she was done (ya a huge singlr pup in the morning one morning proved us wrong. Sadly it must have done sonething toher not long after weaning the pup she died. She was a german shorthair ret/ beagle. She looked like a whippet marked like a Gsr.
I would get her in and make sure she was spay or if she has a physical problem.