I have had the Mirena since July and only had my period once. I also noticed that I pee a lot more often and the urge is stronger too. I too sometimes question about being pregnant. I know a lot of these things are side effects, but it sure beats trying to remember a pill everyday with everything else going on. The Mirena is supposed to be really effective. Was your son born naturally or was he a c-section? My Dr. said since my daughter was a c-section there is a slight chance of it shifting, and had me in a cpl weeks later for a ultrasound. She did say that this was a VERY SLIGHT chance, and many times you feel really crampy if it shifts so that would be a signal. If you are really crampy, I would call your OBGYN immediately to schedule an Ultrasound. If you still get symptoms and think your pregnant, I would just keep taking EPTs to be sure. You never know, my GF got pregnant while on the pill...
PS - My fiance acts like my oldest's "brother" sometimes too, he loves to tease!