is a condition wherein the uterine tissue is deposited at abnormal sites. This tissue is susceptible to the hormones released during menses. Hence, it will bleed from these abnormally located tissues too. Its very likely that you have this abnormal tissue in your knees too, though very rare.
An ultrasound or MRI scan of the knee joint during menses would be ideal to establish a diagnosis. You need to consult a Gynaecologist and an Orthopaedician.