No period, but a few small cramps.

Hi!
I’ve had an IUD since 2016, and I’ve been getting my period every month just like I would without one. Every month, I’d get the typical cramps and such right before my period.

I had my last period on May 3rd, with the usual intense cramps, and haven’t had sex since then, but I did have sex before my period was due. But since I got my period, I didn’t see any reason to worry.
Normally, I’d get my period around June 1st, but it’s now June 6th and it still hasn’t come.

I took a pregnancy test on June 2nd and 3rd using my first morning urine, and both were negative.

So why is my period late when this has never happened before?

Thanks in advance!