Meeting acquaintances can be a weirdly stressful experience. Whether it’s somebody you used to work with, or someone you used to sit next to at school for a few brief months, I think there are a few commonalities that can make it stressful.
For one, the context has changed. If the only way that you’ve interacted with someone is in the role of “co-worker” or “lab partner”, it’s going to be a little weird if you encounter them at a bar. A few forced lines of small talk later, you’re back at your table, but then you pass them every time you go to the bathroom, and it’s weird if you acknowledge them, but it’s also weird if you don’t acknowledge them.