The first period you have on Mirena can be a bit wonky . Your uteruscervix is being more active than usual to help shed blood and the new device can make things feel strange for the first period or two . One potential is that the strings are hanging low in your vagina right now and inserting the tampon and then having it move down into a lower position in your vagina is touching and moving the strings around . Little tugs on the strings can trigger cramps sometimes . Have you checked your strings since your period began ? Have they moved up and out of the way or are they still easily reachable ? If your flow is light and you can handle wearing pads , I would perhaps switch to pads for this period , even just to avoid the cramps .