IUD and periods

So, I've had my IUD for several months. I always had extremely bad periods, but the IUD lessened them quite a lot, but now. I haven't had a period since August. I had unprotected sex about 4-5 weeks ago, but I've taken 3 pregnancy tests and all say negative. Is my period just not going to happen anymore? I'm kinda paranoid and really confused. My sister has had the IUD for almost a year now and she says that she rarely will ever get her period and even if she does, it's just a bit of spotting for a few days. Do y'all think I'm pregnant or is it just the IUD messing with my head?