At one point I read up about HYPP online, and from what I gathered, if the horse is N/H or H/H, they often do seem completely normal, but one day out of the blue they can have an attack - the horse could go most or all of its life without ever having an attack, or it could have one tomorrow. There was no way to predict it.
I don't want to get you needlessly worried til your tests come back, but I will say that my one and only encounter with HYPP was a QH mare who was a lesson horse where I used to work. I don't think anyone there knew what HYPP was (I did not at the time). She had been there for years and was completely normal, but had an attack one day out of the blue in the middle of a lesson. They got the kid off her back before she went down, thankfully, but she went on to have 3 or 4 more attacks in the next 24 hours so they put her down :( It was super sad, she was a great favorite among the kids.