What's causing my chronic knee pain?

I'm a young woman in my 20's. I played basketball throughout my teens. I've been pretty sedentary for the past few years...no running, just walking. I had a baby last November. Two weeks ago, I went running/walking, and it seemed to bother my legs a bit. I hadn't done anything since. My knees began to hurt a few days later. Last week I took a Zumba class for the first time, and it aggravated the pain from all the jumping and twisting, so I did a "impact free" version of the dancing. The next day I did 35 minutes on a stationary bike at the gym to begin strengthening my knees. I took a day off, and the next day my knees hurt so bad to even walk. I'm in constant, stiff pain, especially walking and going uphill or up/down stairs. The pain seems to be getting worse every day, but I have no idea why!? It seems to hurt on the inside of the front of my knee, on the inside of the knee cap. I thought I just over did it when I ran, but I've been taking it easy since and they keep getting worse. Considering going to the doctor, but don't want to be charged $1000000000000 for 2 knee braces and to be told to lose some weight (duh, that's what I'm trying to do, but the pain is making it nearly impossible!) Any advice would be GREATLY appreciated.