My knee hurts on the back side when i fully extend my leg. the pain is greater after workout. what should i do?

Knee pain: Persistant pain in the back of the knee can come from many things ranging feom bakers cyst, tendinitis, or cartilage or menscus abnormalities. U may be best served by getting yr knee looked at by orthopedist.

Seek an evaluation: Your symptoms are nonspecific. In other words you need to have a more detailed evaluation. You could have a cartilage tear, a cyst or even knee cap problems.