Hello and welcome,
Baker cysts are not uncommon and can be caused by virtually any cause of joint swelling (arthritis). The excess joint fluid (synovial fluid) bulges to the back of the knee to form the Baker cyst. The most common type of arthritis associated with Baker cysts is osteoarthritis.
You might ask your Dr. if you could have either ultrasound, injection of contrast dye into the knee followed by imaging, called an arthrogram, or the MRI scan if you want to get to the bottom of this concern.
Baker cysts often resolve with aspiration (removal) of excess knee fluid in conjunction with cortisone injection.
I do understand how tough it is when you do not have medical insurance as these tests get expensive however, living in pain is not allowing you to live your life the way you want to and I hate to read how much pain you are in.
Know I am here and that I wish you the best.