I had unprotected sex and started my period the next day(4 days early)& then 2 days later took plan b, my period has been longer than usual, normal?

Probably: Plan B involves use of hormones for suppression of pregnancy. Bleeding when withdrawing hormones, like in a regular birth control pill cycle, is usually what occurs and may be heavier since Plan B has a higher dose of hormone than low dose birth control pills.

Plan B side effects: are a potential reason. Potential side effects w emergency contraception includes menstrual flow changes (spotting, bleeding, early or late period).