Confusing period and situation!

[innerdimensions]

Hello, im new here...47 and a half years old, on mirena IUD and use condoms as a backup. Had an "accident" 2 weeks before my period, however, i havent been regular in a few years so no idea how long my cycle is anymore as it can be 27 days to 47 days long. Anyway, this time my period started 31 days in but ive barely seen any red blood...its mostly brown, and not very much. Im TERRIFIED this could mean pregnancy, but mirena IUD is supposed to be 99% effective, plus, i supposedly only have a 3-4% chance of even getting pregnant at my age. But still, im very worried...and stupidly, im too scared to get a test done but may have to