Why do you have a sour taste in your mouth when you vomit?
Acids taste sour.
Vomitus (stomach contents that are expelled) contain not only the food that we have eaten, but also mucus that lines our stomach, pepsinogen (an enzyme that breaks down proteins) and hydrochloric acid. One of the characteristics that we use to determine if something (edible) is acidic is by its taste - acids taste sour. Think lemons, vinegar, apples, tomatoes, and raspberries - we find all of these things as being sour. Hydrochloric acid in our stomach is the same, except much stronger (pH of 0.5-1.0).
In contrast, bases - those substances with pH greater than 7.0 - taste bitter. (If you put a small amount of baking soda on your tongue, you'll have a good idea of what "bitter" is like.)