Firstly, wear a HRM. It's hard to tell what's going on inside without knowing HR while you're riding. After you've used one for a while, you'll know about where your HR should be for various efforts. Then on a hot ride, if your HR starts climbing more than it should, and especially if it stays high when you stop, you're dehydrated and you need to drink more. If your HR is like 120 when you're stopped, it's getting into the desperate zone and you need to sit in the shade and drink until it comes down. Don't have to be indoors, just shade. If you haven't peed for 3 hours, same. No pee, sit in the shade and drink until you pee. Not thirsty? Take more electrolytes. I use Endurolytes, 1-4/hour, depending, enough to cause thirst. Thus the more I drink per unit time, the more Endurolytes I have to take to keep myself thirsty. They also prevent sloshy stomach. In fact if I get sloshy, I take a couple of Endurolytes and sip water until I feel it empty. I can even be climbing while I wait for that good feeling. Usually sloshy stomach is from eating too much or eating stuff other than carbs.
You should have been fine to ride after your break, or would have been if you'd drunk enough water to have had to pee. The other possibility is that you were low on fuel. Many times riders get low on fuel in the heat because eating doesn't even sound good. For that reason I drink almost all my food on the bike, a maltodextrin and whey protein mix, 750 calories/bottle. Nothing leaves the stomach as quickly as malto. If you have an HRM, you'd notice HR drop when you're low on fuel. Of course when it's hot, that doesn't work as well.
I also separate my electrolytes from my water because there's not one answer for how much electrolyte and how much water. It'd be incredibly frustrating to me not to be able to have water and electrolytes separate. Also in hot weather I always wear a 2 liter Camelbak and carry my food bottle and usually a second empty bottle in my cages. The empty is handy for refilling the Camelbak and can be filled with water too if it's going to be a long way to the next water. If I'm going to be out in the wilderness with a long way between stops and there might be stream water, I also carry a Steripen, 6 oz. and quite small.
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