Hello, Plan B pill are emergency contraceptive pills, which has levonorgestrel, the same compound which could be found in your birth control pill. This pills works the best if taken within 72 hours of unproteted sex. The most common side effects of this drugs are headache, dizziness, nausea, abdominal pain,breast pain and most important-menstrual irregularities. Its very common that this pill may cause you menstrual changes like spotting or bleeding before your next period.Some may have heavier or lighter period or period which is early or late. Plus side effects of birth control pills are pretty much the same. So I guess, heavy bleeding what you are experiencing is because of the side effects of one of this drug or combined effect of both. However if you continue bleeding heavily for more than 7-8 days and if you experiencing any kind of severe abdominal pain than you should consult your gynecologist and discuss about the same.