pregnancy but got period??!

I recently went to get an IUD at PP and they had me pee in a cup for testing. I'm in the room talking with the nurse and she comes back and tells me that I am pregnant and I can't get the IUD inserted. Mind you I had just gotten my period 6 days prior and it ended the day before. This period wasn't just light bleeding and spotting, it was full on medium/heavy bleeding, I was changing my tampon every couple hours.I also had cramps, was also seeing blood clots when changing the tampon. This period went on normal and lasted 5 days. Now I did have a blood pregnancy test done same day, and will go back in a couple days to retest and see if hCG levels are going up or down to full determine if I'm really pregnant or if it was a false positive. How is that I got my period and everything but got a positive pregnancy result?? If anyone has experienced this please let me know and help a girl out.