AITA for taking back the beer I bought for a party by not chilling it?

This isn’t a super long-term grudge, but more of a “why am I being made to look like an asshole?” situation. <br><br>So last weekend, I was supposed to attend my girlfriend’s friend’s party. I have known this friend for years so it wasn’t a big deal. I offered to grab his beer of choice (Michelob Ultra) and grab pizza. I was gonna have one too, so it wasn’t like I was spending a ton of money. <br><br>Well, I get to the party, there is no one I know there except my girlfriend. I immediately figured she invited me so she wouldn’t have to attend alone. Fair enough. The party was small, I brought the beer and pizza and had one of the beers while there. <br><br>The next day, my girlfriend tells me that the host was upset with her because I ate one of the beers, and she said I “had no right to open it”. Huh? I opened it to bring it, and had one too. I didn’t drink the whole thing, it was full when I left. Why would anyone care if I opened it if I am bringing it back? I didn’t waste his beer. <br><br>I don’t think it’s that big of a deal, but am I an asshole here?