Some of the commentors mentioned that TheAnchoress should kick her Diet Pepsi habit. I had decided to leave a comment on that regard, and then it got too big, so a trackback will do.
I was violently allergic to MSG when I grew up. Any trip to McDonalds, and many of the meals in the school cafeteria, would give me a migraine. Almost everyone has quit using MSG, so my sensitivity to it has gone down a lot in the intevening years. Aspartame is sufficiently similar to MSG that I'm also allergic to it. I would get headaches and other light symptoms every time I ate or drank NutraSweet products. I also hate its taste, so it wasn't hard to not use the stuff.
A couple of months ago, I started taking the low-sugar version of SlimFast without reading the label. I react badly to stress. I am a person that no one, even me, likes to be around. I can't stand anything, and lash out almost without provocation. Since I've started with Current Employer, I've only had the lightest of stress, and can deal with it. Except when I started taking that SlimFast. Then the grouch was back. By the first week, it was rough, but I wasn't getting my traditional headaches so I didn't notice. I skipped the SlimFasts a couple of days and noticed that I wasn't grouchy-bear any more. Two cans of diet formula quickly entered the trash can.
Aspertame metabolizes into methanol, which metabolizes into formaldehyde. Worse, aspertame deteriorates rapidly under 130 to 140 degree temperatures, which aren't hard to reach in shipping crates and trucks in the summer, producing formaldehyde in larger amounts. Aspertame and its children can also cross the blood-brain barrier. There's widespread speculation that Nutrasweet is the cause for the large growth of Parkinson-like symptoms in young people and "Grave's Disease", among other issues.
I was going to suggest that TheAnchoress look at sucralose (Splenda), but the health troop are starting to point out its chemical similarity to DDT and other chlorocarbons. Research it yourself, but don't just jump from Aspartame into Splenda.
Stevia has a fan club, but it tastes metalic to me, and a few people are countering the "all safe" designation. Saccharin appears to have weathered the "rat bladder" problems, but I still don't trust it, and it tastes metallic to me too. I think that Americans need to retrain themselves to a sweet-free diet. Given that I am obese, I know it's easy to say and hard to do... :)
TheAnchoress, drop the Diet Pepsis now, drink water (and for a while, a lot of it) and light juices, and get better. We can't take too many light posting days. :)