advice pls

Ok ladies and guys, need your opinion, this week I wrote an email to my bf told him th at the kids and I want to come home,he replied I needed to prove myself to him if we were to get back together, even tho I have done everything I could.... skip to today I had an appt to with my ob to make sure everything waa ok to try and conceive I got a mag from my Dr, says I have an STD, I know for a fact I haven't been with anyone since we broke up, so how should I tell him that he gave it to me?