For 3 weeks now i have severe pain in my knee when going up stairs or or straight inch my leg. what could this be ?

Knee or back?: You need to consider if it is a primary knee problem or a back/disc problem with radiation of the pain to the knee. I you have back pain with radiation into the thigh that stops at the knee, you would see a neurosurgeon for possible nerve root impingement in the back. If the pain is just in the knee and in particular if you have had a history of injuries to the knee-see an orthopedic doctor.

Many causes: Many possibilities. Often related to knee instability. Have knee medial-lateral stability checked. Then ? Rheumatologist or orthopedic surgeon.