My body retains heat very well. Good for winter, bad for summer. I've had heat problems in the past, even spent the night in the hospital once. Since then I've learned what weather is dangerous for me, what clothes to wear, how to "read" my body, how to pace myself, what foods to eat, etc. I'm having very little trouble in this weather this summer. Your feelings of very nauseated and being bloated are to me classic symptoms of heat exhaustion. In my experience, this feeling comes on in a very short time with no warning - fine now, 15 minutes later laying on the side of the road knowing I need to drink more and simultaneously knowing I can't because it won't stay in me.
What do I do? Learn what type of weather can get me in trouble (You got started on knowing that today.). I wear wicking jerseys, not cotton or wool. I start slower (for maybe a half hour) to figure out how the temperature, humidity, intensity of sunlight and my body are all interacting. After that I can push it harder but I pay attention to my "core temperature". I never push as hard as I can, or as hard as I would in cooler weather. I make sure the breeze or a shady route keeps me cool enough to dissapate my body heat. I tend to stay on flatter routes. Bigger hills will push my body temp. up quickly. If I do ride them I make sure to spend enough time cooling (my body temp.) down before pushing hard (speed or hills) again. Something I'm doing this year is drinking lots of water - probably twice as much (or more) than what you said above. All the above comments on electrolytes, Gatorade, etc. are good information and advice.
If you (or at least when I) got into the nauseated condition you describe, I've learned how to recover (it's a long process). First I just layed in the shade. When able, I started sipping water - very small amounts at first. You want it to stay in you. Eventually (after an hour?) I was able to take in fluid more regularly. I also knew I needed to eat, but what would stay in me? I've discoved grapes work for me - liquid, sugar, and small. I could eat a grape, then a few minutes later eat another, then another. Over an hour it'll start adding up. After an hour of two of resting, sipping water, eating grapes, I was to the point I could eat more (carefully), drink more and start riding slowly, assuming no one had come to get me by that point.
Know what weather has caused problems in the past. Pace yourself, dress right, get acclimated to the weather/heat at the beginning of the ride before pushing harder. Make sure whatever is cooling you is balancing out the heat you produce. Drink lots, and if you're out more than an hour or two, consider electrolyte replacements as posted above. Prevent heat problems or exhaustion, because it comes on so suddenly without any warning. Learn you body's warning signs, and also learn what to do to start recovering if you end up with heat problems.
Hope this helps. Sorry it's so long.