How To Treat Baker's Cyst At Back Of The Knee?

Hi Doctor, i have a movable lump on my left knee after falling about 3 wks ago. I fell on the same knee back in Oct. It hurts if I touch it and I definately can't kneel on it. I have psoriatic arthritis on top of this. Please telll me if I should see anM.D. or not. I have a Baker's cyst on the back of this knee. Thanks alot for your answer....Karen