Well...since you chose a Mirena IUD, I am going to assume you are not personally against the abortificiant action of the Mirena. As long as you personally are okay with that, then it seems like your BC method is working for you, but it seems like your body is trying very hard to conceive. Are you perhaps close to the weight limit for Mirena? This makes it more likely that the Mirena will NOT inhibit ovulation, therefore conception can occur more often, and then the secondary abortificaent action of the IUD takes over.
Also, that last year or so on Mirena, the pregnancy rates go up, again ESPCIALLY for heavier women, so you might want to consider getting it replaced (or whatever your plans are) around 4 yrs instead of 5, becuse your chances for a conception that "sticks" are gonna go up.
Another concern is that...*IF* one of these conceptions "sticks" you are more likely to have an ectopic pregnancy, and/or complications due to conceiving on the IUD. An ectopic pregnancy can be life-threatening and can reduce future fertility by as much as one-half, so this might be an important consideration.
I'm not arguing against the IUD at all, heck, i'm a proponent, LOL, but just want to make sure you fully understand the risks.
Obviously, if you or dh decide that you are uncomfortable with what is likely happening on what seems to be a fairly regular basis for you, you might want to re-consider you BC choice.