Depending on whom you ask, most artificial sweeteners leave some sort of discernible aftertaste. Splenda seems to have the least aftertaste, in my own experience. However, I stopped using it since I have read research that shows that it is harmful to the natural bacteria that live in our guts. Depending on what you mean by your stomach doing "flip-flops," it's possible that the Splenda is throwing off the balance of bacteria in your gut and that is causing problems for you.

As for a sports/rehydration drink, I have long recommended Vitalyte (formerly called Hydralyte and formerly-formerly called Gookinaid). You'll probably have to order it online from their website, although I have seen it sold at REI. You might be able to find another merchant that carries it on their website. It's an excellent sports drink with no artificial sweeteners. I'm reading the label right now--glucose and fructose (glucose is not sweet, which is the reason for the fructose). It does have an allergy warning that the glucose is derived from corn, which may or may not apply to you.

I also drink this when I am sick. Sometimes your body's natural ability to absorb water can be compromised. Have you ever noticed having diarrhea for some reason, like maybe a stomach virus or eating spoiled food, and if you drink a lot of plain water, it just seems to "run through" you? A rehydration drink helps your body absorb the water in cases like that. It can make a noticeable difference in how you feel when you're sick, from my own experience.

Although you didn't mention it specifically, fat substitutes do a real number on me. I'm specifically thinking of Olestra. I tried some Pringles potato chips made with Olestra--actually, I think it was this small, free can of Pringles that came in the mail or something a number of years ago. Anyway, I'm never going to touch anything with Olestra again.