Here are the symptoms, guess the cause (heat?)

Hi everyone,
This forum has always been my go-to place for
training theory questions.

I was wondering if anyone could
comment on the following:

I was out in the mountains today and had a curious thing happen to me. A few hours into the run I started to feel lightheaded, close to fainting. As I continued on I began to feel close to vomiting. I retched once,
but didn’t barf anything up. I had to stop on a boulder slope
and catch my breath multiple times. At this point my HR wasn’t
working by I could feel my heart pounding in my chest.
Couple of details: the run was 25km and 1600m of gain. I’ve run todays route multiple times and have not had too much difficulty with it.
I’ve had this nauseas feeling happen once before on a really hot day. I tried a 100K in 30 oC weather. In this instance I was pretty sure I over exerted myself. I think I got heat exhauastion.

Whereas, in the above example it was much cooler; around
18oC. The weirdest bit was that the nauseos feeling passed and I was able to continue.

TL;DR when out on a long run in cool conditions, what is the
most likely cause of feeling nauseas/fainting?

Thanks in advance

Here is a link to the run https://strava.app.link/sFoi5tpfMBb