I have always been in pretty good shape. I have been a moderate to heavy lifter. For the past month though, I decided to change myself to a long distance runner because i want to run a half marathon. I have been losing weight very quickly and I know some of it is muscle loss during the transition, but I noticed stretch marks near my arm pits this morning when I got out of the shower, and 2 small ones on my abdominals. I dont really know where stretch marks come from but I have always known them to be on fat people. Can anyone explain why I am getting these things all of a sudden?