So I had Nexplanon (not an iud but same hormones as Mirena) and didn't have a period for about a year and then started IF and keto. I lost about 15 lbs in the first month and started having irregular bleeding. My Dr. (who is v cool w/keto diets, she has type 2 diabetic patients going off insulin w/keto) said that when women lose fat quickly it can release estrogen stored in those fat cells which can cause irregularities in your cycle and that should go back to normal when youre at a healthier weight, and that the birth control shouldn't be any less effective. It was a bit inconvenient but yay fat loss! It should become more regular or go away again when you get to a healthier/maintenance weight.