How Should Calf Muscle And Knee Pain Be Treated?

I have pain in my upper calf muscle and back of the knee. It's been 3 days, I just woke up with it the other morning. I haven't been able to walk for the last few days and the pain is getting gradually worse. I went to urgent care tonight and they wouldn't treat me but suggested it may be a blood clot. They didn't seemed too concerned. Should I go to the ER?