I would need your height to calculate your BMI and to see if your weight corresponds.
You have to go for a knee X-ray to rule out osteoarthritis.
If you had any recent injuries then go for an MRI to diagnose any ligament tears or bursitis.
Apply ice packs and take intermittent pain killers and follow up with an orthopedician. Treatment of knee bursitis depends on whether infection is involved. If the knee bursa is not infected, knee bursitis may be treated with ice compresses, rest, and anti inflammatory and pain medications.
Wear a knee band and do not exert yourself till diagnosed properly.