Hi and welcome! I cannot speak to mirena as I have never used it, but I can talk about my experience with Nuvaring, which may or may not be similar to what can happen with Mirena. So IDK if this will even help! Anyway, I was on NR before my first baby for like 7 years. Once we decided to start trying, I removed it the day it was supposed to come out, got AF, and then immediately got pregnant during that first cycle of no BC. So for that reason, I believe you could def be pregnant. My second baby it took me about three/four months off BC to get pregnant, but I had super irregular cycles (some really short and some really long), so that could also be the issue you are having now. Then my third baby I was off BC for several months but wasn't TTC, just relying on pull out method, but obviously that didn't work as evidenced by my beautiful baby boy :-) But I did have very regular cycles during that whole time, at least in terms of when AF showed up, so coming off BC didn't always make me irregular. But anyway, I think it's hard to tell what your true cycle is right off BC because potentially your body is in an adjustment period. So you may not actually be "late" you may just be having a long cycle, which def happened to me. Or you may be pregnant! So IDK if I have helped at all, but you should def POAS! Keep up posted and good luck!
DD 1 (6), DD2 (4), DS (2), MC 7/13