Hi, Plan B has a progesterone called levonorgestrel. Being a hormone, this levonorgestrel can alter the menstrual cycle s rhythm in a very unpredictable manner. As you have taken near the end of your cycle. it is most likely to be your normal periods that is compounded by decreasing levels of levonorgestrel. This might be the reason you are having more clots and blood. If the bleeding and clots continues, do contact an obstetrician. I would recommend you to avoid taking emergency contraceptives as it can result in altered menstruation and also has a higher chances of failure. Opt for a regular contraception or barrier protection instead. I hope the above information helps you. Thank you for writing into healthcaremagic. Have a wonderful day.