Pulsating in bottom. Face with a doctor.

Hi, I am 28 years old I have been facing pulsating in bottom alot like 3 4 times a day and is very intense and gets me very upset. Another strange part is when I masterbate (I am engaged but not married) this pulsating and other weired thing in my bottom stops for sometime. When there is some semens comes out of my penis, I can feel pressure and stress in my bottom. I have been facing this for about 10 12 years and gets me very upset It make me worried that I may have some sexual disorder or if I am lucky this may be some disease related to anus. Please any clear idea what this could be I have tried to talk to a doctor but this isn't very easy to explain when you are face to face with a doctor. Thank.