While there are a lot of aspies who self-medicate with drugs and alcohol in an effort to feel "normal", you don't sound like one of them. There's nothing wrong with enjoying a drink or two alone. There is no physiological difference between relaxing by yourself with a beer, and drinking the same beer around some other people. I would only be concerned if you were binge drinking, and that's if you're with friends OR by yourself.
I can sort of understand your mother's dilemma though. My grandfather had problems with alcoholism for much of his life, which may have contributed to his current health problems, and for that reason I often worry that I'm making my grandmother uncomfortable whenever she sees or hears about me drinking. I was kind of obsessed with alcohol for a while, more as a "special interest" than a genuine addiction, but I've had enough bad experiences with hangovers and just feeling like s**t after drinking too much that I can't see myself drinking enough on a regular basis to be an alkie.
But my point is, your drinking habits are nothing to worry about. If anything, I should probably be a little bit more concerned about mine, and they're nothing compared to what a lot of young people around here do.
Don't forget, most social norms are created by NTs, and generally, NTs prefer to do things in groups rather than alone. The idea of enjoying anything alone is weird to them.