Is it possible to be selectively allergic to something?

My only allergic reaction ever was to these damn things.

No rhyme or reason. Only thing I have ever been allergic too.

Could be that all the chocolate milk providers get the chocolate part from the same provider.

Have a look at the list of ingredients. I looked at them for Yoo Hoo (the only american brand I know of) and there is a whole host of other things in there. Any one of those could be an allergen.
INGREDIENTS
water, high fructose corn syrup, whey (from milk) and less than 2% of: cocoa (alkali process), nonfat dry milk, natural and artificial flavors, sodium caseinate (from milk), corn syrup solids, calcium phosphate, dipotassium phosphate, palm oil, guar gum, xanthan gum, mono and diglycerides, salt, spice, soy lecithin, niacinamide (vitamin b3), sucralose, vitamin a palmitate, riboflavin (vitamin b2), vitamin d3

In brief, no. There must be something else in it.

It’s possible that your reaction is something akin to muscle memory, in that you were violently ill after drinking chocolate milk as a child or baby, and your body/mind makes the association and expels the milk.
Founder: to become ill from overeating.

See also: Psychosomatic Disorders

Lots of people are psychosomatic when it comes to food allergies. Thinking they will be sick makes them sick. I know of a woman who claims to get I’ll from eating anything she thinks is GMO. It is a form of eating disorder in her case because it really messes up her life. She is obsessive.
More people have something resembling your trouble. If we’ve been sick on something we learn an aversion to it. Mine is strawberry shakes.
Often, something we thew up will put us off it. It really is a clever evolutionary adapted trait. Probably saved our ancestors from eating bad food.