std with condom ???

I had sex about 3 days ago, we used a condom. But I had gone to get tested about 5 days ago, yesterday they called me and told me I was positive for chlamydia. Is it possible I could have transmitted chlamydia to the guy even tho we had protected sex?? 
(My and my ex broke up about a month and a half ago since I found out he had been cheating on me, but didn't go get tested until a few days ago..)