Hi there, I have understood your concern and I will suggest you the best possible treatment options.
First of all do not panic.
You might be aware that emergency 
contraceptive pills like tablet PLAN B, contain high dose of 
progesterone hormone.
It helps to prevent unwanted 
pregnancy in most of the cases, if taken within 72 hours of unprotected sex activity.
Secondly, the withdrawal bleeding that occurs after these pills is quite erratic.
It can be early or late and it can be scanty or heavy.
Your abnormal bleeding can be due to plan B pills.
Your other symptoms can be due some kind of bowel infection, or may be due to anxiety about pregnancy.
For
Please seek prescription support from your treating doctor for tablets containing 
Ofloxacin and 
Ornidazole combination twice a day for 5 days.
I will suggest you to get proper sex counseling done about safe and protected sex activity. This will help to prevent unwanted pregnancy and STDs.
I hope this answer helps you.
Thanks.
Dr. Purushottam Neurgaonkar