I was convinced that i had it. Because when i was like 5 or 6 i got really sick and covered in red spots, and my little sister got them too as we were so close and i remember us being in the bath together soaking in that stuff they use for chicken pox (sorry baby brain).
You could have had it but only have a very low level of immunity built up - they can only test for your current immunity level and most people if they have had it are fine - but some just don't develop the antibodies to it very well. At least you know so can try and avoid it.