I've been breastfeeding Genevieve for almost 7 months now-- YAY!!
Yes, the mirena is a variation on the IUD which is a small insert into your uterus.
I have been feeling the exact same way.
I actually had a period at 2 months PP then haven't had one since. I'm exclusively BFing so I was surprised when I even got that first one. I've wondered when I could expect to get it again. DH comes home in March and I've got no birth control lined up, so we shall see.