My period has always been regular before I got my mirena, and once the mirena had settled down I stopped getting periods at all. Now, over the past couple of months I am getting a period again, roughly at the same rate as pre mirena. I have no idea what's going on either! I mentioned this to my GP and he had no clue as well. So I can't directly answer your question, but I wanted to let you know that you're not the only one with period weirdness happening
(and btw, I've also noticed that the week or 2 before my period my weight is likely to go up or stay the same, instead of trending downwards - this has been responsible for my frustrating stalls, but it was only once I'd had a couple of cycles that I realised what was going on).