This is not implantation bleeding. This is breakthrough bleeding caused by taking Plan B. It is bleeding between periods. You are not pregnant. The pill worked to prevent pregnancy.
Plan B prevents pregnancy by preventing ovulation. Your next monthly period might start earlier or later than expected. Flow might be lighter or heavier than normal.
Anonymous, you are young and have made the decision to be sexually active. Please start using some type of birth control. An emergency contraceptive is designed for emergency only situations and is not intended to be used as birth control.
Please learn about all options:
"Which Method of Birth Control Should I Use? Part I"
"Which Method of Birth Control Should I Use? Part II"