break up and back together with STD

My bf and I have been together about 3 yrs then broke up for 2 months and now have been back together for 2 weeks and I just found out I have chlamydia. I don't know if he gave it to me or I gave it to him? I'm scared to tell him (yes I know, I am going to tell him). I think he has been with atleast 6 girls since we broke up and I was only with 3 guys. I really think he gave it to me, but I know he is going to think the opposite. How do I tell him and how do we find out who got it first?