It could be a nerve, arthritis or hip problem causing the tremor. If it keeps happening, get medical help. A vet will likely have to x-ray to make a diagnosis.
Do you see you dog favoring the leg at all while running or jumping? If you do he could have some type of injury. It could be a pulled muscle or something going wrong in the joints of the leg. You might want to go get it checked out with the vet to rule out an injury.
Dogs usually give birth 63 days after becoming pregnant. Pregnant dogs may have made up their own minds about where they will labor and deliver their puppies.
It may just be because the muscles are tired--think about how sometimes after exercise, human muscles feel weak or shaky. If you are worried, you could always ask your vet at your next visit.
