Behind my right knee felt swolen, like a small waterballoon. after 3 days the pain was worse, causing me to limp. it went away but comes back often and i am no longer comfortavble with extending my leg out- because of the pain?

You need to see an Orthopedic MD for diagnosis and Rx. In the meantime wear an elastic knee support available in most stores or pharmacies, a heating pad few times a day and ibuprofen for pain

Start with your primary care doc for an exam. While the history suggests a popliteal cyst, other possibilities cannot be ruled out. Your doc will be able to further coordinate tests or a referral based on actual findings.