The symptom I get almost immediately after taking any form of Aspartame (even when I don’t know I’ve gotten it) is nausea. The degree of nausea depends on the amount of Aspartame digested. One of my co-workers baked a delicious cake (we have cakes at the office the first Wednesday of every month). About 15 minutes after eating it I started feeling real sick to my stomach and couldn’t figure out what was wrong. Later I heard her tell the recipe to another co-worker, the icing had NutraSweet in it. Diet drinks do the same thing to me.
Aspartame is also a common migraine trigger (recognized by most headache research centers as such) and I get migraines.