Plan B and weight?

So I've read on multiple different sites and even on the back of the box that plan B doesn't work if you weigh over a certain weight! (I believe it's 176 lbs) do any of you have any insight on this? Because I've had it happen two times now that I've taken plan B (I weigh about 210lbs) and I will have a random period about two weeks after my real period. And by reading how plan b works, it makes me guess that I possibly had an egg fertilized and it didn't take hold in my uterus, therefore forcing my body into having an extra period that month? I've been on a ton of different birth controls, including two IUD's, implanon, various pills.. The IUDS fell out of place, the implanon was an absolute nightmare (literally I got diagnosed from PTSD/anxiety after about two months of having it in) and every pill I've taken has caused terrible side effects. So I'm basically forced into only using condoms and trying my hardest to track my period and not have sex on my ovulation days.. As much as I hate to use plan B sometimes I have to. Has anyone else had a similar experience with it? Anyone over the weight limit? Because I usually have a super timely period regularity.. Just curious!