What Causes Vaginal Odor While Having IUD?

I have been having to change my underwear twice a day lately because it smells really bad after I wear it for a few hours, but I went to my ob/gyn and my swabs were clean (I thought it was a bacterial infection). What could it be? I am over weight, could it be from sweat? I also have an IUD, could that have a part in it? Should I buy all new underwear? THANK YOU.