I was doing some research and I found your answer. But you're not gonna like it..
It's called "levator syndrom". It's caused by a number of factors; sexual activity, fatigue, lower back pain, bowel syndroms, prostate disorders, or even direct contact to your coccyx (tail bone).
The reason you wont like this is because the treatment does require hospital attention. I havn't found anything that gives a home solution.
The treatmen goes as far as a doctor sticking a finger in your rectum, and other means of discomforting anal treatment.
I'm 24, and I get it too. The pain is seriously overwhelming. Maybe in six years I'll get a prostate exam...