IUD "Working" or Pregnant?

So, I got my IUD (Mirena 5 year) in place early April, 2016, and I've had random bleeding/spotting since I had it inserted, and last had bleeding mid September, and now I didn't get my period for Oct 3, like I should have. 
Is there a possibility I could be pregnant, or that the IUD is working completely? I will take a test here shortly but I wanted opinions first. 
-- Please note I am in a completely monogamous relationship, hence why no condoms are involved. --

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