Levonorgestrel
"Hi all- hoping to impart some peace of mind to anyone waiting for it to work. My boyfriend & I had sex on May 4th, last day of my period. He finished in a condom, then later went back inside me unprotected. He didn't finish the 2nd time, but I wanted to be extra careful. I took Plan B ~18 hours later. I had a withdrawal bleed five days later (May 11-12th) that lasted 2 days, and was supposed to get my period on the 27th- it came today, 6 days late on June 1st (stress/jetlag may be to blame). Some bad side effects- depression, mood swings, thick white discharge, headaches, fatigue. 
I'd urge you to keep track of your cycle for added reassurance in these situations (know your fertile window, etc.). Sending peace & patience & don't be scared!"