I don't drink things with Aspartame because it makes me feel queasy. I don't know of any mechanism that causes that effect. Occasionally I encounter something that I would not have expected to contain Aspartame that I notice the feeling before I have even considered the possibility that it might be present. I take that as a sign that it is not psychological.

Same here. Very little amounts already give me a weird tummy feel. A normal amount (ex. Half a can) gets my tummy turned around for a few hours.