I think it's verging on malpractice to place a Mirena in someone so young. Progesterone birth control is bad for teenagers because your bones are still strenghthening. I know the DR. will say it stays in the uterus, but that is NOT TRUE! All of us on here would agree that the Mirena hormones went into our bodies. Please get it out.
I did try the paraguard before having children (which is not exactly recommended, either) and the main side effect I had was bad cramps during my period and random pinching. I think there wasn't much room in there since I'd never had a baby.
Good luck to you, and let us know what you decide.