Why do I feel pain at my perineum area after ejaculation? Am 20 years old male, it is like a muscle is over tense or something... Seems like a cramp

See your doctor. Problems like these can only be correctly handled by your doctor in person. He/she needs to listen to you, perform an examination and possibly run labs or other tests. That's the only way he/she can find out what's going on and what to do about it.
Well. Some can get a perineal cramp, give urself a break, could be prostate, especially if u get pain when u pee or fever.
Muscle contraction. When you ejaculate you bulbocavernosum muscles contract. These can sometimes cramp and cause that pain. It is not dangerous unless you are having other symptoms.