Several reasons. You had it but he never caught it (not likely if no sex with others in past few mo). His test missed it. His infection cleared on its own (gon is cleared by the immune system in a few wks or mos). He was treated, didn't tell you. Or he isn't being truthful about his test result. Most important: he still must be treated. NO sex with him until he has been treated.
See below. The risk of transmission of n.Gonorrhoeae from an infected woman to the urethra of her male partner is 20% per episode of unprotected sexual intercourse increasing to 70% after 4 or more exposures. The risk of male to female transmission is approx.60% per contact. Did you have relations with another person?