Hi, I have lumps behind my knees. My right one is larger then left. Size 2inches be 3 1/2inches on the right. Left half that size. Very painful.

Cysts. These lumps may be baker's cysts and can be best evaluated by an orthopedic surgeon and /or an ultrasound. Since you're experiencing discomfort, I recommend that you look into checking these lumps out by a specialist.
Baker's cyst. Have you had them for a while? See your doctor or a sports medicine or an orthopedic surgeon for this. You might need either injection or physical therapy or even surgery, depending on how they respond to initial treatment. Meanwhile, you could take some ibuprofen or naproxen (if no allergy) and do icing and compression wrap.