I've read it's because your body has spent the past 8+ hours trying to lower your core body temperature to prevent overheating during the race, and once you stop racing it has to reverse course and warm you back up.

 
https://www.triathlete.com/...-after-a-race_320486

I get this frequently after long, hard efforts (training or racing). For me it kicks in about 15-30min post race and last a couple hours. Sometimes I'll have my early morning fleece from setting up transition on as I'm packing things up post race and it's 90^F out.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Yes it happens. Take a hot salt bath.

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https://www.strava.com/...tes/zachary_mckinney

FWIW I've had it three times --

once after a longish Tough Guy race with lots of immersion in January

twice after childbirth. The docs said it's quite common.

so . . . a regulatory mechanism, but of hormones, electrolytes, breakdown byproducts, temperature, who knows . . .

I've gotten the post-race chills/shivers after each of my last three IM distance races, all of which went better than the first three IM's.

I don't know if it's a function of being able to push it harder for the duration of the event, a function of age (it first happened to me in my upper 30's), or something else.

But I've just come to expect it (as has my family) and incorporate laying under blankets in the medical tent for 30+ minutes shivering, drinking, and recovering.

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Tad

It took awhile, but I finally discovered that its not the destination that's important, but rather the journey.

I Get them most IM that aren’t hot. Eating ASAP after the race helps alleviate them for me. Hot bath also. Will last for quite a while.

After watching Eric Engel bundled up post IMWI this year, I have started packing a winter clothes bag for post race. Thick fleece lined pants, warm outer, stocking hat, gloves, and wool socks. My family brought it to the finish line at Louisville and it sure beat those stupid mylar blankets. It is also a good use for the IM bags we receive at registration.