Ok wtf.....

Before I was with my boyfriend of 6 months I was tested for every STD possible. It was negative. But as soon as we started having sex I was Tested again at my annual appointment. It came up positive for chlamydia. He was the only person I was with...... but he just got tested and they are saying his test is negative for everything according to him............ This makes no sense. Can any one explain how this could’ve happened