I know the feeling. One day I am happy, and care free, and then bam, I feel "off" and very confused as of why this happened and all I want to do is either stare at something or at the computer. In my case it kind of spiraled into an obsessive thought. I am going to the mental health doc on Tuesday to figure out what might be my cause.
Has this happened before? I am not a psychiatrist nor psychologist but maybe it's clinical depression, sometimes clinical depression doesn't have to have environmental triggers. If you can rule out that it's not any signs of a physical problem (ex. thyroid, hormones, ect.) I would suggest talking to a therapist about it and see what they say. It is better to get it checked out now to possibly prevent further happenings from getting worse.

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