Missed period - what do you think?

Ju
Hi Ladies, 
I'm on the IUD, and in the beginning, I didn't have my period at all, which is quite normal with Mirena. After about a year, it returned, and started to get predictable again. 
Now I missed a period, and I'm sort of getting worried...but it's a chance of 1 in 1000, so am I overthinking things?
Thx (: