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

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