Why am I randomly depressed?

Some days I'll come into work pumped and ready, and others I feel like I don't even belong on the same planet. Almost like everyone is against me, talking about me behind my back, like nothing I do is good enough, and anything I say is either not funny or falls on deaf ears. But I know that all of this is just in my head which is the unnerving thing. Currently I'm lower than low and I can't explain why. I'm not sure how to diagnose myself, if I need help. I'm not contemplating violence I just would rather be locked up in my room away from everyone else. Even the things I have fun with are bland and uninteresting. This seems to happen every other month or so, any guidance would be most appreciated. Thank you in advance.