I had 1 emergency pill on 16th Aug. My last menses 7-11th Aug. Since today morning I am bleeding with cramps. The brownish blood flow is very thin.

Plan B side effect. What you described is likely a side effect of the emergency contraceptive pill. You may consult this site for more information.
http://www.webmd.com/sex/birth-control/plan-b#1
If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD.
Practice safe sex if you have sex.
Get HPV vaccine, if you are younger than 27 years.