For newly heavy periods, a few different things may be going on. You may have uterine polyps, also called endometrial polyps. They’re growths in the uterus usually about the size of your pinky finger or smaller. They can peek out of your cervix or stay tucked inside your uterus. These polyps can cause your periods to get heavy and can cause bleeding between periods. Or you may have uterine fibroids. These balls of muscle tissue in your uterus start off as small as blueberries but can get as big as softballs or even larger. Fibroids keep your uterus from contracting well during your period, so you end up having heavier bleeding that lasts longer. Or you may have adenomyosis, a cousin of endometriosis. With endometriosis, tissue similar to the tissue that lines the inside of your uterus — the endometrium — grows outside your uterus, most often on your ovaries, fallopian tubes and along the pelvic walls. With adenomyosis, small deposits of tissue burrow into the walls of the uterus and cause heavy periods.