I am posting this to possibly keep someone else from getting nailed. I'm 51 & have worked outside my entire life & never had this happen:
I went down in a more controlled manner at the back door, crawled to the fridge, grabbed a cold drink, & had to rest for 5 min before making it into the shower. I ran cold water over me for 20 min or so.
You are very lucky....I'm glad your OK!!!
I have collapsed 3 times by being in the heat.... I'm 42 and i sweat real easy when I'm working in the heat, and i try & drink plenty of water & iced tea.
Last year(age 41) it was very hot & dry here... I was baling hay & it was in the upper 90s with high humidity and i was also pushing myself hard when i got dizzy & blacked out in the field.. Lucky i had someone with me and they rushed me to the hospital, and they told me at the hospital that i was real lucky.
Another time when i was 39 and picking blackberries on a strip-mining job.. I was on a ATV and miles from the nearest house, and i blacked out... I had a small fan mounted on my ATV and i was able to get to it and run the fan on me to help cool me down, and i was sick for days after that..... This also happened to my father-in-law while picking blackberries.
And another time was when i was 33.. I was having my new home built and i wasn't even doing the work, I was just sitting in the sun watching them... I started feeling funny and stood up & blacked out then collapsed, and off to the hospital i went.
After 3 different times of blacking out & collapsing in the heat, the hospital finally ask me last year when this happened if i was taking any kind of
medication.... I had been taking
"medication for nerve damage in my arm"
I found out that
WHILE TAKING THE MEDICATION YOU SHOULD AVOID THE SUN. 
...The doctor that put me on the medication never told me to stay out of the sun.

.. and this could have easily killed me the first time, and i don't take that medication anymore.... Anyone taking medication should try and find out ALL SIDE EFFECTS of medication.
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(You said that you ran "cold water" on you for 20 minutes.)
I have always been told that if you get really hot from being in the sun, that you should not get in a cold shower to cool off... I was told that
when your body gets really hot and then you get in cold water that your body can cool down to fast, and cause your body to go into shock, seizures and other dangerous side effects.
If i get real hot, I always try and cool down first before i take a cold shower... and also if I feel like I'm getting to hot, then i avoid going into the AC to cool off to quick.
*Thanks for sharing your story! Hopefully it will help someone!
Hope your feeling better today!