I agree that it was probably all the sugar/carbs. Are you diabetic or is there any chance you are prediabetic?
The only other shot in the dark I could take is perhaps it was too much sodium? I’m assuming you’re adapted to Soylent’s low sodium, high potassium mix. Did you notice any swelling, particularly in your feet or lower legs? I don’t know if that could explain the nausea, honestly, unless you were also dehydrated (which sounds unlikely after drinking so much soda).
I found this under causes of tinnitus on WebMD:
Other medical conditions such as high blood pressure, cardiovascular disease, circulatory problems, anemia, allergies, an underactive thyroid gland, and diabetes
I honestly don’t know if a high sodium intake can cause acute high blood pressure (I would assume so?) or if it’s more of a chronic problem with high sodium intake.
Out of curiosity, any reason you mix them? I think I actually prefer diet soda to regular soda now, but I recently tried Dr. Pepper 10 and I think that’s my new favorite (≈10 calories from sugar per 12 fl oz). I think they also make 7UP 10, but I’m not sure what else. I don’t think it’s the tiny bit of real sugar that I like so much as a better mix of artificial sweeteners (I think they use acesulfame potassium in addition to probably decreased aspartame). I’m wondering now if it would be feasible to add some ace K to normal diet soda (Dr. Pepper 10 is usually sold out at my local grocery store), but I bet it would be too sweet.
The only other thing I could think of was caffeine. Do you normally ingest caffeine? I think going from no caffeine to too much caffeine in a short period of time has made me feel not-so-great before.
Oh, and if you don’t need the carbs, the sugar free sports drinks aren’t bad at all. I prefer them, but I’m not a runner so I usually only drink them when I’m (very) sick. It feels like years since I’ve had to, thankfully.
Best of luck to you!