What Causes Pain On The Calves While Doing Exercise?

I recently started training to run a 5K race. About a month ago I started feeling pain on the inside of both of my calves. One morning it was so severe I could hardly walk home. I took two weeks off to let it heal and bought new shoes. I started running again last week and the pain is back. Again today I started to run, but after only 3 min it was so bad I headed home. What would be causing this pain? What can I do to alleviate it and prevent it?
I am 34 year old female, and I am overweight.