I have had the same problem, 10 lbs. since November. I also have been blessed with a high metabolism and 4 consecutive years of nursing. My second child is cutting back now that he's two. I had an IUD put in last January; not Mirena, but the copper one with no hormones. In my case, I don't think the weight gain is from the iud. But it is frustrating since I've never had to worry too much about it. I'll be 40 next year so I guess age is catching up with me. :)
I know with birth control pills, weight gain is sometimes a side effect of the hormones. Maybe Mirena uses the same hormones.
Another factor might be simply that you are a busy mom with 2 kids now and you are not as conscious about your eating habits as before when you had more time to think. lol
Sorry I don't have any better advice. Good luck