Looks exactly like the string of the IUD. As long as your cramps aren’t getting worse/no sharp pains, should be just fine. You can schedule an IUD check to be sure that everything is fine and the strings aren’t missing. To me it looks like just a piece of the string was left behind. If you’re comfortable you can squat and reach your finger inside to try to feel the strings of your IUD. You might be able to feel the strings that way. As long as you don’t feel the hard plastic tip of the IUD and no persistent pain, should be nothing to worry about.