Hello, and I hope I can help you today.
First I want to give you some basic information which may provide you some reassurance.
For one, it is extremely rare for a woman to ovulate during her period, unless she has very short menstrual cycles. So it was doubtful that you would have been fertile at the time you had your condom accident.
Secondly,
Plan B is effective in preventing
pregnancy 9o percent of the time when taken within 72 hours of unprotected sex, however it can still convey some protection for up to 5 days after the episode of intercourse.
Thirdly, it is a common side effect of Plan B to have spotting four to 10 days after taking the medication. This is not a period, it is just triggered by the drop in hormone levels after the Plan B is out of your system.
The most common sign of pregnancy however, is a late or missed menstrual period, so if your real period is late this month, then I would perform a pregnancy test. A pregnancy test will become positive within 3 weeks of conception, so if you had sex on the 16th, any time after the 27th or so you could check a pregnancy test and if the results are negative, than you have nothing else to worry about.
I hope I was able to adequately answer your question today and that this information was helpful.
Best wishes,
Dr. Brown