If you only bleed after having intercourse, there may be something else causing the problem, if not the Mirena iud.

The home page for Mirena states this:

"Bleeding and spotting may increase in the first few months and continue to be irregular. Periods over time may become shorter, lighter or even stop."

Other extensive patient information at their site states this:

"Changes in bleeding. You may have bleeding and spotting
between menstrual periods, especially during the first 3
to 6 months. Sometimes the bleeding is heavier than
usual at first. However, the bleeding usually becomes
lighter than usual and may be irregular."

Still, if the bleeding only happens after sex, you should have an examination done by your doctor to determine why; to find out if the iud is in position and the area healthy, and make sure nothing else is causing this problem.

Merely Me, a writer and community leader here wrote an article titled, Bleeding Between Periods: What Does It Mean? which might help too.