you should check the thread in your vagina to rule out any misplacement and also go for a pelvic ultrasound scan to rule out any damage to your uterus.
It is likely just from the Mirena. Because it is hormonal, it can cause various effects including irregular bleeding/spotting. Watch the flow over the next few days and see what it does. If pregnancy is a concern and it's been two weeks since you last had sex, take a pregnancy test with first morning urine to see what it shows.