Chlamydia

Okay so a while ago my boyfriend and I had been together for 4months and he tells me he is going to the doctors. After his appointment he rings me to tell me he is being tested for chlamydia....... He tested positive and then so did I. Is it possible he got it from his ex who he slept with before we got together or was he definitely cheating on me with her?? He had no symptoms until this time and neither did i