Reason For Weight Gain Near Lower Abdomen And Stretch Marks?

Hi,
Recently I've gained weight around my lower abdomen, to the point that I have really dark stretch marks. I'm worried that there might be something wrong, as I haven't gained weight anywhere else and this has occurred over a few weeks. I have cut out carbs and sugar from my diet to attempt to lose weight, but it hasn't helped with the swelling/weight gain around my abdomen.
I've spoken to my doctor but he said not to worry about it - but it just doesn't seem right.