Does anybody have any clue what this is?

I have the Mirena IUD and have horrible cramps all the time. I am going to get it taken out soon. Today I was having wayyyy worse cramps than normal so I layed in bed and they just kept getting worse. I used a heating pad for thirty minutes to an hour and it started helping the cramps. I turned it off and got up to take a shower. I started undressing and noticed these weird marks on my stomach. Does anybody know what this is? It's kind of freaking me out and I don't know if I should be worried or not.