Yes i understand, it makes sense. But from what you say I feel there might be another explanation. Even without preparation, I can easily hold my breath for 50 s without really starting to gasp. This means I would start to faint even later. So if you don't know if you held your breath for longer than 15 s, it means it was probably less than a minute (right?) and if this is correct I don't think you would have any reason to faint from lack of oxygen. But what could definitely happen is that you have a sensation of fainting because of anxiety or because of that panic attack in the first place, which is an extremely common thing. There are countless people who suffer from panic attacks and who feel they are fainting because they feel they are having a heart attack, or because they are dying, when their only problem in the end is "only" the panic attack they are having. Because panic attacks alone do that too.
Personally I do feel my breath, but I have felt in the past that my body did not belong to me, or that it wasn't me speaking, so I can definitely imagine that breathing could feel different. I have tried just now to switch from automatic breathing to controlled breathing but the limit between the two is not very clear to me. I have had unrelated problems where it was hard for me to take action about something, it felt like I could not start the action, and then I would start to do it kind of automatically at some point and I didn't know why. So it was a kind of problem between auto and manual mode, it is annoying but it isn't something I worry about. One doctor told me they call this ambivalence, but I havent heard people connecting it with dpdr.