Why would somebody who used to be able to drink with the best of them now get a headache after a single beer?

Good question. No obvious answer. Maybe it's an allergy to the gluten or other contents within beer? Some people get migraines when they start to relax. Could this be it?
Oh, yes..... Your body gets used to higher blood alcohol levels after enough time and exposure. This isn't a good thing because the damage and death levels don't change, meaning you can get pretty close to danger when getting chronically drunk. However, if you allow your body to "heal" over time, you become intolerant of even low levels of alcohol exposure.