What causes the feeling of sexual frustration?

I used to think it correlated with the semen and prostate fluid being pent up in your body but after starting estrogen to stop my MPB I still experienced the pent up feeling even though my testicles produced almost no sperm.

What's the biological explanation of this? Is there some kind of horny timer that makes your body want to have a sexual release even in the absence of sexual bodily fluids?