i got the mirena put in 6 weeks after my son was born. it was fine at first, but then i started experiencing horrible side effects:
dizziness,
couldn't lose weight (i was up every morning at 5:30 at the gym, eating right and taking good care of myself, so i was doing everything right)
my periods lasted for 2 weeks straight, sometimes longer.
depression/over emotional
anemia
so i got it taken out after having it for a year and my side effects were gone within a week.
but... i didn't get pregnant on it :)
you might not get the symptoms i had on it, but if you do get the mirena and experience anything like that or that's abnormal for you, then you're not going crazy! i thought i was going crazy and that something was wrong with me. my doctor didn't even think it had to do with the mirena until i actually got it taken out.
on the pill now and i love it :)