Well to start off with, there were 3 things that really did not help with my mood in the first place.
Ok, first of all, I am seriously not a party/gathering person. I really don't like being in large groups; I think the most I can tolerate is a group of 4 or 5. Any more, and I start to feel kind of pissed off. Secondly, I don't do well with change. In this case, my friend invited a lot of her new uni friends. Finally, I was in charge of getting part of my friend's present, and it was sold out at the branch that I went to. It was pretty last minute, so I didn't have time to go to another branch to find it. So, yeah, I was quite upset at myself.
In addition to these three elements, hormones kicked in and I started overthinking stuff about my unrequited feelings for one of my friends, how I've hardly made any new friends in uni compared to some of my other high school friends, etc. Aaaannnnnndddd, queue the tears.