How To Treat A Pulled Muscle Of A Swollen Knee?

Hi, I have been doing a lot of running and other exercise recently and been fine, but whilst walking yesterday, felt a pull in the muscle/ tendon at the back of my left knee.. thought I could walk it off and went to an exercise class this evening and now my knee is swollen and it's quite painful to walk! Any advice, please? I am a 33 year old female, about 2 stone overweight, 5'7'', no medical problems.