Plan B is a pretty high dose hormonal birth control pill. All the side effects of birth control pills are thus associated with Plan B. Normally you will see bleeding about a week after you took it. This is not your menstruation but bleeding from your uterus wall because the Plan B hormones are no longer present. Your real menstruation can follow this at the normal expected time or later or earlier.
Is this perhaps Plan B bleeding combined with your own menstruation?
If the bleeding does not stop or get heavier, see your doctor.
Take care!