Here are the possibilities, assuming of course that everything you said about yourself being a virgin is true:
1) False Positive (rare)
2) False Negative (rare)
3) He had chlamydia but treated it with meds so it didn't show up on the test
4) He had chlamydia but it went away on its on after a few months
That's it. No other possibilities. Think about each of those four and ask yourself which one is most likely for your situation.