Can you get drunk from one beer?

I am a 21 year old female, about 175 pounds. I don't drink often, in fact I've had one Bud Lite in the last three months. I got drunk for the first time on my 21st birthday and don't remember much of it, but of course I did polish off half a bottle of vodka. Usually it takes two or three screwdrivers OR two Black Russians (my two favorite drinks) to get me drunk. On a recent occasion however I chugged one Budweiser (celebrating the death of bin Laden) and became very light-headed and disoriented, not to mention quite numb all over, as I usually do after a couple of stronger drinks. Since I chugged it, and since I don't usually drink beer, was I drunk? I am not a heavy drinker, and I am just curious.