What Causes Heavy Bleeding 2 Days After Periods After Taking Plan B?

I had sex the last day of my period. Took plan b the next day. Experienced a very heavy flow of blood 2 days after. Breasts are very sore now. Last period began on march 27th. End date on April 1st. Plan b taken on the 2nd. My question is what was that bleeding All pregnancy tests were negative. What is going on? I am 23, weight abI had sex the last day of my period. Took plan b the next day. Experienced a very heavy flow of blood 2 days after. Breasts are very sore now. Last period began on march 27th. End date on April 1st. Plan b taken on the 2nd. My question is what was that bleeding All pregnancy tests were negative. What is happening?