Leg pain that started at the knee?

About two weeks a go I worked out for the first time in a long time and everything went fine. I didn't fall or twist my leg and I haven't banged into anything. I didn't hear any popping sounds either. The next day (almost 24 hours later) out of no where my knee started to hurt. It started on lower inner side of my leg and when I bent my knee it went in a diagonal pattern to my outer lower thigh. There wasn't really pain in my thigh but more of a tightness. After about 3 or 4 days my leg was much better (80% - 85%), then out of the blue it got worse. Again no injuries during this time. Now it's not just in my knee but the back of my knee, back of my thigh and calf. I can still feel tension in my lower outer thigh just above my knee whenever I bend my leg. I've always been able to walk on it but since this has been almost two weeks now I'm becoming concerned that something is really wrong. What ever I did surely should have healed by now right? It feels fine when I sit down. It's only when I bend my knee or walk that I feel the pain in my knee and possibly other places (calf, back of knee, thigh). Anybody ever had anything like this happen?