It sounds like an infection
Antibiotics may help but a vet will confirm with You the results
Why wait ?
Go on Angie's List and go to a vet that is highly rated and will help You Out fast, efficiently and at a good rate too !
Other than take her to the vet, try a Google search. The vet would be a better option of course.
From what you described, it sounded like a blister of some sort. Maybe you could put a gentle antiseptic healing cream on it....(something you may have in the medicine cabinet) and see how she is in a few hours. It may also be nothing to worry about. Things always seem worse in our minds than what they really are, especially when we love something like our pets. I hope she will be alright.
drain the anal sacs