Before going too far into physicals, doctors and nutrition and so forth, consider that it could also be a simple matter of conditioning. I have experienced the same thing (still do) when I push hard outside of my comfort zone. It doesn't matter what the distance is either, or how fast, it's a matter of my current fitness level and what I tried to do. If I feel slammed after the ride, 30 or 40 minutes later a feeling of lassitude and want to take a nap, it's because I pushed beyond what I was used to. Also if I got hot and dehydrated, but you said that didn't happen.
That can be tested IMO. Just do the same or similar ride the next week, and the week after, and if the physical symptoms lessen or disappear then congratulations you're more fit. You might need more miles in between though, to hang onto whatever gains you may get from one workout.