Post exercise blues

Been having some bad days recently. Despondent and out of sorts. Itchy feet. Unhappy. I've been putting it down to a sense of boredom and feeling the need to broaden my horizons. Someone at work today said she knew exactly what it was. Post gym blues. She said I complain of this on the days after I've been in the gym. Post seretonin come down or something. Apart from the health benefits, I always knew the gym was bad for you!!