Are you vaccinated or have you caught it previously (or both)? It’s entirely possible you’ve been exposed but your body fought it off before you could harbor enough to test positive.

We had a houseguest test positive (after previously daily testing negative ☹️), and I was definitely exposed. I felt under the weather for a few days but never actually tested positive. According to what I read in the NYT this means I successfully fought it off. I’m fully vaxxed/boosted.