how could this happen

im am 16 years old and just found out that i have Genital Herpes he was my first and still is but we could not understand why i have it he was clean and like i said he was my first if he had it he would of told me about it. when i got my first breakout he got his first one a day early when we did not see eachother two or three days before. my mom told me to break up with him but i will not because he did not know he is not one of those people that dont say anything he is realy honest person who sometimes can be an ass for being to honest.