Thanks for replying, guys!
Seems that passing before treatment happens more often, but we all end up looking very young. I think that is why these two fellow students were asking.
I think the entire encounter took me off guard, which is why I had mixed feelings about it. I just did not expect to pass just yet and most students will assume I'm a hardcore lesbian or something. Last year I had two encounters like that, which were awfully akward:
When waiting for a boat to do fieldwork on an island, a class of schoolchildren (I think they were around 12) arrived. A group of girls were whispering and pointing at me, and then the cheeky gits came over, going like "Are you a boy or a girl?!!!!". Again, it took me complegely off guard and I asked them to guess, but they really had no idea. In the end I just gave them an ominous answer, but really, I was shaking afterwards.
When doing my job on that island (catching and bloodsampling birds with a fellow student, a girl), we were sitting on a fence, and a tourist approaches us. He was clearly curious about what we were doing (cycling around with a ladder on your shoulder gets attention) so we explained the whole ordeal. And then out of the blue he asks: "So you are here with your girlfriend?" Both the girl and I went wide eyed, but I loved it! Awkward, yes, but worth it.
I also got Sir'ed a few times on the plane but seeing that I was sitting, and it was darkish, it's easier to mistake me (or actually, finally getting it right) for a guy.