I am experiencing knee pain and knee weakness.

In one's mid-30's...: Might be just an overuse injury or other inflammation, but an exam is needed first. One can start with her primary care doctor, who can refer to rheumatology or an orthopedic doctor if needed. A rheumatologist knows a lot about arthritis. A physical medicine doctor may also be helpful. A physical medicine doctor is a physiatrist, knowledgeable in nerve pain, muscle problems, and physical therapy.