It is a condition of Phemosis. The foreskin in such a case is tight and prevents smooth retraction onto the penis. While masturbating, because one is in self control of stimulation, there is not much of a discomfort. However, during sex this is not possible and the discomfort is severe. It can also cause loss of Erectile due to pain while sex. If not corrected, it can predispose towards penis infections.

Your symptoms are suggestive of a condition called phimosis. By the time most uncircumcised boys are 10 years old, they can pull the foreskin back from the head of their penis. For some, it may not retract completely until they are about 17 years old. When this happens, it’s called phimosis. I would advise you to visit a urologist so that you can be assessed and the treatment can be decided. There are 2 main treatment options mainly by medicine where you use a topical steroid cream and loosen the skin or by surgery by full or partial circumcision. If the problem persists or worsens visit the urologist at the earliest.