I agree with Thunder Bow. Recently my doctor reduced my antidepressant by half. After a month, I noticed this kind of sad and bad thoughts popping into my head uninvited. I recognized this as a symptom of depression and went back to my doctor to have the med readjusted. I'm already feeling better. Some of us just have a chemical imbalance in the brain that requires a bit of tweeking.

I should add, I dealt with this mindset all my life until being diagnosed with depression and getting help, thru counseling and, most helpful, medication.