Yes. It is absolutely the Plan B pills. One of the side effects of Plan B is irregular bleeding. Plan B is a hormone pill with many times the amount of hormones in a normal birth control pill. It works by suppressing ovulation, thickening the cervical mucus, and making the uterus unable to support implantation.

Since the sudden rise in hormones from the pill is then accompanied by a sudden drop, bleeding occurs. This is what happens with regular birth control pills, but Plan B is an extremely high single dose. Bleeding is common, and Plan B could mess up your cycle for potentially up to three months.