A couple of my roomates/best friends from college live in a nearby city. I'm in medical school, so I'm quite stingy with cash. One of my old roomates is also in medical school, but with a substantial scholarship and the other roommate has a nice paying job (like 60-70k). Earlier this month they asked me to come up and visit and hang out with them on a Friday night. They told me they were gonna get wine and cheese and we were just gonna hang out. I wasn't sure if I would be able to make it, and they checked back multiple time to see if I could come (which I really appreciated). When I got there, I drank one glass of wine but brought the rest of the alcohol I drank with me. I had a good amount of (nice) cheese and crackers as well that they bought. We had a great time and it was awesome to see them for the first time over a month. 

However, at no point leading up to that night did anyone ask me to buy in or mention what it would cost. I even talked with them while driving to their place and they didn't mention it. The day after, I got a venmo request for over $16 by my friend with the job. I was caught off guard, because I thought they had invited me as a guest and weren't expecting me to pay. I really don't mind paying my fair share, but since they didn't mention so much as a price beforehand I thought they were just being nice. I figured my friend would be aware that I would need to know this beforehand since he knows I am super poor right now. I haven't talked to my other friend who is also in medical school about it, so I don't know how he feels about it or if he even knows the other one requested payment. I paid the request, but politely mentioned that in the future he needs to let me know if he wants me to buy in as well as what it costs, but he brushed it off. I'm not like angry or anything, just annoyed. I feel like when you invite a guest to something you were already planning, the norm is to not expect payment, especially without warning?

AITA? I honestly don't know and I feel like I'm being petty about it (this post feels ridiculously long). Thanks for the feedback.