Hoping you have figured this out. I experienced this with the Mirena IUD, slowed down my exercise and then ultimately had it removed. I had a uterine ultrasound and low and behold my uterus has a unique shape (no uncommon). I am no longer eligible for IUD options, due to shape not leading to effective birth control effect of IUDs. Once removed, I am now fully pain free and can push my body as hard as I want, not possible with IUD in place. I know others who have had pain with exercise with normal shaped Uterus as well.
Beth (Family Medicine physician)