I'm not sure what's causing your knee pain because everyone is different but after 13 knee surgeries I took up riding a little over a year ago. Luckily my ortho is also a biker and gave me great advice...keep riding because it strengthens the surrounding muscles which help keep your knees working and be aware of your knees - the instant you feel a ping, twinge or stab drop gears even if it's on the flats. Every day is different for all joints and muscles so what gear you ride in one day or what speed may not be good for another. Oh- and popping two extra strength tylenol before the ride isn't a bad idea either!