Lucie, you said yourself that it's listed as a side effect. Therefore, that is what's happening here. Everyone knows that taking the plan b pill messes up their cycle, it's pretty much common knowledge. I wouldn't worry about it at all. It's too be expected after taking that pill, so just relax and you'll be fine.
Umm i hope u check to see if your pregant or get tested. There are preiod tracker apps so you have less chance of having sex during the time you can get pregant and maybe it was just rough sex?
I took the morning after pill and it made my period a week late!