Is this a little below her anal opening and off to one side?
Or is this along her abdomen between her hind legs?
This might be a problem with an anal gland that is infected and near to abscessing open to drain.
You can read about anal glands here
http://www.marvistavet.com/html/body_anal_sacs.html
and see a diagram of a possible abscess here
http://www.vetwest.com.au/files/images/Anal%20gland%20diagram.jpg
You can try a warm damp compress on the area to see if that lets it open and drain which will reduce the dog's pain but it will need a vet's attention and likely antibiotics.
If this is a bulge in her vulva and she is not spayed and is coming in heat the hormonal changes may be causing her to swell there and sometimes a dog can have a prolapse of the uterus or vagina which will make a bulge in the vulva area.
One more thought if she is not spayed and was in heat 2 months ago then you may be seeing a bulge from a puppy that is stuck along the birth canal if she got loose at any time while in heat. That too would require vet care.
As she is in pain depending on how you feel about the local emergency care you may want to see them to find out just what is going on and how best to resolve the problem.
Hope this helps you!
Because it could be an infection I'd not suggest you try to empty the very sore anal gland though a vet may be able to do it without the dog biting him or her.
If you suspect from the above info that its an anal gland issue then seeing a vet and getting her on antibiotics etc would be a good choice.
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