Splenda causes problems for me, too. Horrible, sickening headaches that OTC medicine won't touch, dizziness, nausea, rapid heartbeat, and feeling like I'm going to pass out. It very nearly sent me to the E/R once. Luckily the effects went away in a few hours, but I was still unsure what caused it. Splenda was a suspect, though. A few weeks later, I accidentally drank part of a glass of Kool Aid that had Splenda in it, and started having the same problems. That's when I figured out what was causing it. You really have to read ALL the labels. They put Splenda (aka sucralose) in everything these days, seems like ... even some medicines!
I stick to Stevia whenever I can, with aspartame as a second choice. Aspartame causes reactions in some people too, but so far it's never bothered me.