This is what my lifelong "overheating" episodes are like. I was wondering if any of these look familiar to you. All of the following happens instantly after exerting for a short period:
- Feel very overheated
- Intense pressure/burning sensation in head and ears
- Heart feels "thick" and is suddenly double(?) speed
- I feel wierd irratic heartbeat flutters
- vision starts greying out, feels like I'm going to pass out
- people that have seen it say I turn grey/white
- Have to stop everything, subsides between 10 seconds - 1minute.
- Have to avoid "sudden"/intense exercise like sprinting... have to work up to it
- Have learned to feel it coming on and avoid it most of the time

The first most severe episode I remember is passing out while running in 7th grade gym class. Despite this I've carefully exercised for many years.