I have a copper IUD (ParaGuard) and I feel pregnant?

I had a ParaGuard copper IUD inserted the end of November of 2010. I have had no problems with it other than a normal increase in cramping and bleeding during menstruation. This month however I have completely missed my period. My last period was 5-6-11 and I have a 28 day cycle (my cycle has been an expected 28 days since the birth of my daughter three years ago).I had a few days were I noticed some pink tinged discharge when I went to the bathroom at the time my period would normally occur but I never actually started my period (I have excessively heavy periods). This week I have noticed pains in my breasts, kind of like cramping when your milk lets down while you're breastfeeding. My nipples also seem to be a bit more sensitive. I also noticed yesterday that I was experiencing a tightening in my uterus (the only way I can describe it was that it was a tightening like Braxton Hicks contractions) that came and went over the course of about an hour. Today I also feel slightly nauseous but not too bad. I can't buy another pregnancy test until I get paid on Thursday but wouldn't a positive result have shown up last week? I have done internet research on amenorrhea and a copper IUD and that is not common at all. Why would I have missed my period when I'm regular like clockwork?