Late period and IUD question

I'm 3 days late on my 2nd pp period. My first one came on time and was as expected. This cycle I only have unprotected sex just before the end of my period and just at the end of my cycle, the day before I was supposed to start my period. All other times were protected. I'm supposed to be getting my IUD placed today, they originally were going to reschedule me for next week incase. I called back to ask because it feels like a complete missed period or it's just coming late and what happens if I miss it next month and I'm not pregnant as they like to place it between 1-10 days of your new cycle. They said they will book me in today and urine test me and if comes out negative they will place it but it's to my own knowledge that there is no way that I could think of that I could be pregnant, they want that certainty and I've always tested just before my period and it's always been accurate so their test should be accurate this afternoon. Anyone else?