I post this here because I think it might be something some of us can relate to.
Even when I'm happy, thoughts will just randomly come up and bite me in the ass. I could be doing something completely innocuous like brushing my teeth and then my mind will go, 'hey, remember that time you farted in front of class?' or 'hey, remember that <insert mildly triggering event here>?' and I'll feel terrible, but it'll just go away after I ignore it, and won't come back.
It's been doing that lately, and it turned a simple thought into a huge to-do. I thought about nearly everything horrible that my sort of girlfriend and I had ever done to one another, and that made me a ball of unhappiness, because I let it take over me, and ruminated about it.
Does anything like that ever happen to you?