Gosh, it's tricky sometimes to figure out. I'd do the progesterone test over again. My mother had this situation and it was related to her progesterone. Are you hoping to get pregnant as well now?
After you do the progesterone test, if it doesn't indicate an issue, ask for a uterine MRI. Sometimes tissue is left in the uterus after a pregnancy and causes scar tissue. If no other (hopefully simpler) answer is found, you'd want to rule that (and fibroids) out.