you need your doctor to check you out. possible you might have arthritis. this can cause a lot of pain which your knee accident could have started. there is medication for this. you are so young. but it could happen. ask your doctor. he needs to check the veins in back of your knees. if your doctor is no help go to a new doctor. don't wait too long. ake care of your self. mandy876
Could be a pinched nerve or fibromyalgia. Might want to see a neurologist.
I would advise you to see your doctor who can examine your leg, listen to the blood flow and also do a Doppler test to rule out any deep vein thrombosis.
It may be referred pain from your knee problem or if you have lower back problems, but it is better to get it checked out.
Best wishes.