Today I ventured out for a relatively short ride and experienced some very strange/scary fluctuations in my body temp in the middle of the ride. I wanted to see if anyone else has experienced this or might know what it was.

As a little context, this is my second season on my road bike, and I have about 1700 miles under my belt so far. I'm 28 and healthy.

I set out to do about 30-35 miles this afternoon under a relatively hot sun (heat index of 93 degrees). I started out pushing pretty hard because I have been feeling good lately (I finished back-to-back 75 mile rides two weekends ago, both in under 4 hrs; a couple days later did a fast 20-mile ride and two days after that a 13-mile sprint).

The first 10 miles were moderately hilly, and I averaged in the 21 mph range riding solo with a slight tailwind. As I hit the top of a hill at the 10-mile mark, I decided to take a short break. I drank some more water and ate half a powerbar. About 90 seconds in to my rest, I could feel my body temp falling, so I got back on the bike, fearing I was letting my heart rate fall too far.

However, I could not get back into any kind of rhythm or seem to get my speed back up over 18-19 mph. My legs didn't feel fried, but I did start feeling a bit nauseated - though not dizzy - and chilled. Before long I was getting goosebumps.

At this point I started getting concerned, so I headed home and took the rest of the ride pretty easy. Still, the goosebumps never really went away (AND I continued to sweat) and I didn't start to feel capable of pedaling hard again until the end of my ride (mile 19-20).

Any ideas?

My hypotheses so far:

-I just bonked (though why the drop in body temp?)
-I was not well hydrated/nourished enough going into the ride (i.e., I bonked)
-I rested too suddenly - without warming down - after being at a peak heart rate
-I was riding too high above my target heart rate for too long (wish I had a heart monitor!)

Sounds to me like heat exhaustion. I get that during the summer. Usually I can negate the effects with electrolyte replacement and drinking LOTS of water. These symptoms can be serious. Be careful out there!

You can go into a heat stroke:
Take it easy in the heat.

https://www.medicinenet.com/heat_exhaustion/article.htm

Ditto, I've had it happen a couple of times. Once, about 3 mi. from home, I stopped, pulled over into some shade, and called my wife as I was recovering. I just wanted to make sure I had her on the line if my condition worsened. I took a couple of minutes, and was OK to ride home. Another time, I did our club century in September (in central NJ!) with a heat index of about 115F. I got home, and my wife just kept me hydrated and cooled until I was fairly recovered.

Don't mess around with this stuff. It's dangerous.

you were not properly hydrated/nourished going into the ride.

Thanks, everyone, for the comments. Heat exhaustion had come to mind, but I expected that I would have been dizzier and more faint if that had been the cause.

I think you are right, though. Some combination of poor nutrition/hydration going into the ride and the hot weather and hard pedaling probably caused some mild heat exhaustion.

Twice I have attempted to ride in temps over 85, and twice I got what I always thought was heat exhaustion. Reading those article(s) linked above, I now realize that the first time was actual heat stroke (second time it was just heat exhaustion). It wasn't even 10am and it was over 100 (this is extremely rare where i live). I finally had to stop. I was shivering, not sweating, hallucinating, very nauseated and had a belly full of electrolytes and water that wasn't doing anything for me. Fortunately I stopped at what turned out to be the start point of a Team In Training event. The two ladies manning the water cooler noticed me huddled on a bench, to weak to even call for help. They immediately went to work on trying to cool me off (ice bags under the arms and at the groin, but not on the head, as that prevents heat from leaving), and called SAG for me. I was SAG'd back, and promptly placed in an airconditioned room. For the next two hours some folks who were volunteering at the start point of the ride stuffed me with what must have been an entire watermelon and a gallon of powerade into me. Strangely enough, since this event was put on by a Jewish school, everyone had had experience with this stuff due to summer trips to Israel. There was also an MD who monitored my vitals periodically, as my heart rate was way up when I was brought in. Fortunately, there were no ill effects.
I was extremely lucky that those two ladies were there where I stopped, or I might not be here today. As a result of these two experiences, I do not ride in anything over the low 80's. It's not worth my health or potentially my life to ride in the heat. In some ways it limits me, but my body just cannot cool down during exertion over a certain temp no matter what I do.

I have felt like that before, usually from not drinking enough water, and it takes a little time to get it flowing back through your system. When its hot, drink drink drink