Whatever it turns out to be, I would go see a doctor to make sure.
Hope this helps
Hi sometimes i have the same problem. I'm a 23 year old woman. I think what you may be expericing is proctalgia fugax. Proctalgia fugax (or levator syndrome) is a severe, episodic, rectal and sacrococcygeal pain. It can be caused by cramp of the pubococcygeus or levator ani muscles.
During an episode, which sometimes occurs after orgasm, the patient feels spasm-like, sometimes excruciating, pain in the anus, often misinterpreted as a need to defecate. Simultaneous stimulation of the local autonomic system can cause erection in males. Because of the high incident of internal anal sphincter thickening with the disorder, it is thought to be a disorder of the internal anal sphincter or that it is a neuralgia of pudendal nerves. It is recurrent and there is also no known cure. However, some studies show effective use of botulinum toxin, pudendal nerve block, and calcium channel blockers. It is not known to be linked to any disease process and data on the number of people afflicted varies, but is more prevalent than usually thought.
The pain episode subsides by itself as the spasm disappears on its own, but may reoccur.
I ot this from wikipedia.