Why am I suddenly really sad, I had a really good day today and suddenly I hate everything?

This happens repeatedly. Everything I enjoy, and it's affect on me, is short lived. I always end up just sad, no matter my circumstances. It's been like this for a while but I think I'm getting a lot worse and I don't know what's happening to me. I'm medicated for OCD and something else I think and it might be the medication but I just don't know anymore. It's gotten really bad these past couple weeks and I constantly feel like I'm gonna do something horrible, tonight is especially bad. I don't know what I'm doing. I do not want anything bad to happen.