I have an 8 month old puppy. The area where she urinates is swollen. What might be the cause of this?

Has she been fixed? If not then she is most likely going into heat. If it is what I suspect then she will begin to emit a bloody discharge for several weeks and urinate often. The best thing to do is bring her to the vet and make sure that there are no other causes for this and to get her spayed as soon as possible.