Hi Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern.
Yes intake of plan B can cause brown
vaginal discharge. This bleeding you had must be
breakthrough bleeding. This type of bleeding normally occurs within 3 weeks of intake of a plan B pill or
emergency contraceptive pill. There is nothing much to be concerned about it.
A plan B pills works by preventing implantation of fertilized egg on the wall of uterus. It is 95% successful if taken within 24 hours of having sex.
As you have taken the plan B pill within 2 hours of intercourse, you should now relax. The possibility of pregnancy is very less in you.
Don't use a Plan B pill too frequently. Use condom or regular contraceptive pill as method of
contraception in future. Regular taking emergency contraceptive pills can cause problems like
irregular periods, ovarian hyper-stimulation syndrome & even infertility.
For more queries consult a gynaecologist
Hope this information helps