What Causes Brown Discharge During Periods?

i had a mirena iud for3 years without having a period. last month my body began rejecting it and i had to have it removed. i barely got a period after it was removed and this month i had all the symptoms of a period but instead of blood it was a sludge like brown discharge, and it only lasted 3 days. ive never had a child and i got my iud when i was 17, im worried it has caused problems with my uterus. is it normal for this to happen?