I would suggest you to rule out any pathology in your legs and then go ahead.
Get an MRI done and diagnose soft tissue problems and discuss in detail with an orthopedician and come to a diagnosis.
Rule out rheumatoid arthritis.
Continue pain killers and supplements and continue your cardio after getting a go ahead from the orthopedician.
I would ask for an MRI to make sure that your ACL and Meniscus is in tact. Now that it is summer, you can try water aerobics and swimming, which is much easier on the knees. Even if it is rheumatoid arthritis, you can still do water exercise. I would also recommend checking in to a community center or health club which may have an indoor pool to use during the colder months. Swimming and water aerobics is a great form of exercise, great for losing weight and good for your heart. I actually find that the pounds come off quicker when using water exercise. Be sure to always consult your doctor before starting an exercise program. I hope this helps you and that you find an answer soon.