If you don't have other health issues, it could be over-ejaculation. Do you ejaculate a lot at night?

Anyway, you should go see your doctor instead of asking a bunch of horny guys and gals on the internet.

Could be how you masturbate. According to an article:   Traumatic Masturbatory Syndrome (TMS) is the habit some males have of masturbating in a face-down (prone) position. Some TMS practitioners rub their penises against the mattress, pillow, or other bedding, while others thrust into their hand. Some rub against the floor.

What's wrong with masturbating in this fashion?

Masturbating face-down puts excessive pressure on the penis, and especially on the base of the penis. These sensations are not easily replicated in conventional masturbation or in sexual intercourse. This can make TMS practitioners unable to have normal sexual relations.

Solution- Stop masturbating for a week or so, then learn to masturbate the right way.

I agree with above poster.  See a doctor who will diagnose the problem and give you a solution.  Do a google search on TMS, but again I stress that you see a professional- a urologist (MD) or licensed doctor.