Its possible that she could have a Inguinal Hernia / groin hernia or possibly even two Bilateral Inguinal Hernia /one on each side. I just had this issue with one of my pups in my last litter. They look like little bumps on one or both sides of the pelvic/inner theigh area close to the volva. The pup was crying out each time she tried to poo and would run around and sometimes even backwards trying to potty. My vet said that it is possible that they could correct themselves and if not will get worse and worse and cause more damage and also that the pup if female should never be breed. If they do not correct themselves in time they will need to be surgically corrected as soon as possible. Which is something that is less costly if done during a spay surgery.
I would ask your vet to check her for this. Its very easy to tell if she has them.
Dog Hernia | Inguinal Hernia in Dogs | What is a Hernia? | petMD