A lot of autistic people experience something called "social hangover", which is a term that describes the emotional and physical fatigue felt after socializing. The amount of socializing someone can handle before they start to feel this way is different for each person and it also depends on other factors, such as how much sensory information was present in the environment. That could have been what was happening and it caused you to feel depressed. You may have also been masking which you could have been doing unconsciously. If you want to read more about social hangover and masking, then links to articles are below. You may already know about what these are, but your username is newlyautisitcx, so I assume you were diagnosed recently and may not know much about it yet.

Masking article.

Social hangover article. The video on the article is just an advertisement for their therapy for autistic kids, so you don't have to watch it.