My dog went lame in a back leg during her heat with no sign of pain

A dog can limp for many reasons: trauma, poor joint development, or post-fracture effects. If trauma damaged a nerve, pain sensation may vanish, so the leg is lame yet painless.

Poor nutrition or genetics can also cause joint dysplasia with no pain felt. If a fracture was not treated in time, incomplete healing can leave a limp. Have a vet check it.