The swelling you mentioned on the area between your vagina and anus (or the perinium) could be due a whole host of factors. The burning upin urination could be some kind of urinary inflammation/infection as christine correctly pointed out.
I would further agree with Christines commect that the doctor is best placed to make a definitve diagnoses through examination. If you are sexually active, then I would probably take a routine urine sample for testing for the common STD's like Chlamydia and Gonorreah.
Hope this helps :)
Adam
Go see your doctor for tests. It could be a bladder/urinary tract infection or possibly something like yeast. Your doctor can do tests to figure out what it is and how best to treat it.