There could be a multitude of thing wrong with her, but with the symptoms your are describing she should be seen by a vet right away before it's to late.
My Dog Has A Swollen Stomach And A Puss Type Of Liquid Coming Out From Her Behind. Her Hind Leg Also Seems Weak And Appears To Buckle. What Is Wrong?
Sounds like an infection in a very serious region. Get her to a vet NOW!
My dog was doing that and if she isn't fixed then she could have an infected uterus. They had to do surgery on her to take the uterus out. Get the dog to the vet ASAP.