My leg always hurts after basketball. what could be the reason why?

Lactic acid: Sometimes when you are either very dehydrated, haven't stretched or warmed up, you can develop pain after a contact sport like basketball. Its something that should be checked out especially if it affects your knee or if you have had trauma to your knee. Also if you are playing bb once a week or once a month without regular exercise, injury is highly likely. Hope this helps!