What Causes Pain Behind Knee And In Calf Area?

HI! So for the last couple of days, I've been having some soreness in my left foot, and then yesterday it went up my leg and now I have a pain behind my knee and in my calf area. I looked after work today and theres a swollen part coming from behind my knee to kind of the side of the top of the calf...probably 3 inches long. It's not discolored, just a lump that i noticed. I am overweight (very) and I work at a job which keeps me on my feet about 30 hours a week. I'm 21, and have recently started exercising and running on the treadmill, but I haven't gone on the treadmill in a couple of days, so I don't know if this is the reason or not. I'm worried about it, and I wanted to know if you knew what it could be and if I should go to a doctor. Thanks Heather