If I have copd, why does one cocktail (alcohol) seem to relax my breathing?

Dulls sensation... Alcohol is a depressant so it dulls your perception of shortness of breath in your brain. You may feel less short of breath but may actually be hypoventilating which could lead to further increases in carbon dioxide. So, you need to be very careful w/alcohol use.
I think it... ...Mostly calms your anxiety about breathing. If you do a pulse oximetry on yourself before and after the drink, you may find the oxygen level in your blood actually dropping.