PERIOD/SEX/IUD

I had a period that lasted only three days but was pretty heavy then I went to the gyncolgesist to get a IUD and before I got it they tested to see if I was pregnant and I wasn't So I'm not for sure right? They put the iud in and I think I'm just worrying myself. I usually have periods that last 5 days so three days was weird and I've heard of people having periods when pregnant so am I just freaking myself or?