I'll be honest - I feel like this might be making a mountain from a molehill.

Don't get me wrong - being stuck picking up after someone who isn't doing their job sucks, especially when you got passed over for their job (and aren't getting paid as a result). However, that happening is just a reality of the workforce, academia or not. This isn't the last time this will happen to you.

To me, this reads like the person who was supposed to do a job didn't show up. That job still needed to be done, so they had to wrangle a few students (or just you) to get it done in the time frame it needed to be done by. Maybe you were picked because the faculty recognized you as someone who stands out, or is friendly, or maybe they just saw you first. The fact that they didn't select the male student doesn't really say anything to me - just that they didn't pick him.

The comments by the center director sound like an acknowledgement and appreciation that you stepped up when it wasn't your job, and helped them out of a tough spot. That kind of recognition is a good thing, and can definitely help you down the road. Unless I'm misinterpreting something, I have a hard time seeing a negative here.

I think you got caught in kind of a cruddy situation, but I don't think it was sexism. As long as you aren't continually being asked to do your classmate's job without pay, or that there isn't a regular trend of male students not being required to help, I don't think there's anything untoward going on.