What Causes Bleeding After Taking Next Choice?

Hi - I am 25 years old. My last period was march 22. My fiance and I had sex on march 30 - he did not wear a condom and I am not sure if he came inside me or not. I took the first next choice about 2 hours after sex and the next one the following morning about 20 hours later (I forgot to take it 12 hours later). About 5 days later I started bleeding - lasted about 4 days. I am 3 days late for my period and have taken 2 pregnancy tests - both negative. Is it safe to assume that I am not pregnant - or could they be false negatives? I am scared and nervous!! Any advice would help!