Please tell me if this is being a controlling friend?

I'm 21 and my friends are 18 and 19. I know I shouldn't have, but I bought alcohol a few weeks ago and shared it with them. I later found out that I can get into trouble by doing that and later that night I got really drunk (8 cans) and ruined the night for everyone. From then on I told myself I wouldn't drink recklessly (I had two shots last night), and I wouldn't buy them alcohol anymore. I learned my lesson and only do it occasionally.

Now, this has caused an issue where my 18-year-old friend thinks I'm being controlling by not buying it, even though she can ask someone else to do it (which she shouldn't). While my 19-year-old friend has accepted it, the 18-year-old doesn't and she continues to argue with me about my decision every now and then. Just now she asked why I took a shot last night at my friends' house despite not wanting to buy alcohol, and I said "I dunno, I just did. No one got drunk and I drink occasionally and in very tiny amounts." Plus the friend I took shots with is 27 and very responsible, as were the other two friends I was with that night. The 18 year old wasn't there.

The conversation ended with me telling her to respect my wishes and that I wouldn't buy them any alcohol period, I stood my ground, only for her to call me controlling. That made me angry because I'm not a controlling friend, in truth (I haven't told her this yet) I'm afraid of something happening while they are drunk because that will fall back onto me for buying it. I care about her a lot and we always remind each other how glad we are to have each other. I have also never given her crap for anything she refused to do, no matter how absurd it sounds, as long as it wasn't going to cause harm I never opposed her decisions. Yet she's furious I refuse to buy her alcohol.

With all of this information, am I being a controlling friend? If not controlling then what am I being? She is my best friend and I really care about her, which is why in my mind I'm looking out for her.
Please tell me if this is being a controlling friend?
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