That'd be my first guess. I didn't have side effects from plan B the one time I took it, but I think it's quite common for it to affect some women's menstrual cycle for the first month or so. It is, after all, a sudden large dose of hormones.
If you take it during ovulation it's still effective as it does things like make your vagina inhospitable to sperm... You should be fine. I wouldn't worry about it and then maybe take another pregnancy test in a couple of weeks to be absolutely sure.