Should i go to the doctors/hospital?

You should have it checked out. Go to a doctor, not the hospital since its a non emergency. You wearing tight pants can have an affect considering with a bruised knee you need to let it be able to move. Yeah all you think it does is bend forward and backwards, but your knee does a lot more. The doctor may look to see if you haven't torn your ACL, I play baseball and it happens a lot. If its major major pain then it could be your ACL. Have it checked ASAP.