Feelings of sadness/ depression without a reason.

Recently, I’ve found myself being sad without a cause that I know of. Like today, for example, I was feeling sad and almost depressed and I felt like nothing could pull me out of that sadness. I didn’t know what was wrong when my boyfriend and everyone else at school was asking. Normally, I’m one of the most cheerful, positive people I know but on some of these days I turn into a completely different person. I did have a small anxiety attack prior to the sadness due to speaking in front of the class and getting some of the questions wrong but that’s not something that would typically make me super upset the whole rest of the day. Does anyone else experience this?