I am on mirena #2. I was training for IMWI (Sept) when I got my first one inserted (June) so I was in the middle of a heavy training block. And like you, I did suffer from minor cramps with exercise or immediately following. This lasted about 6-8 weeks, I think. I know I remembered being a bit worried about it for the race but the cramping resolved itself before that and I had no issues on race day. And no real issues ever since.

I had my second one (5 years later) inserted this past Spring and I had almost no issues what so ever. I expected some minor spotting or cramping and had none.

Edited to add: I am in complete agreement with you on the internet searching on IUDs. There is some scary stuff out there and lots of people who blame their IUDs for a myriad of seemingly unrelated issues. I mean I guess it could cause some of these issues but so could a million other things we do as a normal part of living.