I know this sounds gross but i had anal sex a few days ago and i've been in pain since i think it's a hemrrhoid but can it be serious ?

Sounds traumatic: An anal fissure is like a "paper cut" in the lining of the anal canal. Like a paper cut, it can reopen when traumatized (as when anal sex stretches the canal) and as such will immediately and really hurt. Other possible injuries to the canal: muscle tear, hematoma, abrasion...You get the idea. Get evaluated please, and think twice before re-engaging.

Anal fissure?: Minor injuries are common in anal sex, especially the first time or if not done often. Probably just a minor tear in the anal tissues. Anal fissure (a chronic sore that can follow initial injury) and hemorrhoids also are possible. STDs like herpes, chlamydia and gonorrhea are possible if at risk (new partner, etc). See doc if pain persists more than a week or if any doubt about STD risk.

Can Be: Best to be on the safe side. See the specialist, the GI or Proctologist MD for examination. Some hemorrhoids will go away - especially at your age- but others will remain. Best to get it checked to be sure and treated as needed. Good Luck.