Why would my tennis coach be upset about me drinking a pint of beer with my lunch?

Hey, I'm sitting a PTR tennis coaching course and today was my first day. I was off a very nervous disposition so I didn't sleep last night etc etc. So anyway, I got on pretty well with everybody at the start, including the coach and I was just my friendly self. When we ordered our lunch I ordered a pint, thinking nothing off it... I'm 21, I come from a difficult background, and its my personal choice. Admittedly, I look very young and cute for my age, most people would say I look about 16/17. But I'm not sure that that was the issue. A the end of the lesson he approached me and said "alright, mate, now I don't mind you drinking on a thursday night or whatever, but for the rest of this week I don't wanna see you buying any more pints". I was kinda confused that it bothered him so much... And then I realized that must be the reason he was picking on me a bit after lunch. Can you help me get my head around this?