I've had no problems drinking alcohol in the past. but last week, i had a very bad reaction. how is this possible?

Hard to answer...: Allergic reactions develop against large molecules such as proteins. Ethanol is a very small molecule and doesn't elicit an antibody response, so can't cause true allergies. Most reactions to alcohol are related to a toxic effect as the body tries to metabolize it. Rarely, other substances (like grapes in wine or grains in beer) can cause reactions. See an allergist to help you find a cause.

Etoh: Etoh is a spontaneous mast cell degranulator. Can cause all over flushing and feelings of awfulness. Also, maybe you simply drank too much?!?!