I think if the Dr said not to worry, then you can safely assume that it is just the effects of growing and stretching.

Last weekend I had crippling pains in my lower abdomen, but it was just ligament pain from the baby growing (and too much activity!). I went to see the Dr and he said that pregnancy is a time when women experience all sorts of unusal pains because there is so much happening to our organs, muscles and (pelvic) bones. They really only worry if there's bleeding accompanying pain, but if that has been checked and cleared, then you just have to trust that everything will be fine.

Def. go see the Dr again if there's more bleeding though.