If i didn't have it, and he didn't have it, where did this STI come from?!

My question is, I was tested for Gonnerhea and Chlamydia at the end of April of 08 and I was negative. I just got another test done when I was refilling my birth control three months later and came out positive. I know I surely have been faithful, and so has my partner, but we can't understand where it would have come from between the last test and this test....