You could have the gene for HIV immunity. This is a gene that was in English people who survived the Black Plague. In some small villages the only survivors were people with this gene. They interbred, and the result is pockets in the English countryside of people with a high occurrence of this gene.

HIV uses the same route to get into cells as plague, so the gene protects against both. People with a single copy never get very sick, though they may test positive. People with two copies never test positive. In one case a man with HIV+ got a bone marrow transplant (for another reason) from a donor with two of these genes. His HIV went away, and he started to test negative.

More likely you are just lucky, though. After they broke up with you, they hit the clubs, screwed everyone in sight, and picked up the virus. Exclusive couple sex is a lot safer than random hookups.