This may be a totally stupid question, but have you ever had problems with hypoglycemia (i.e. blood sugar dropping precipitously when you don't eat or you exert yourself)? I have often found myself getting depressed when I exert myself, and I haven't eaten anything or ate the wrong things (i.e. alcohol or candy). It feels like the world is caving in until I eat something. I often have to mentally ask myself if something is bothering me about a situation to justify my reaction. Most of the time it is because I haven't eaten recently. A quick snack fixes it. Sometimes these things are simpler than they appear. The other possibility is a chemical reaction to some medication you might be on. I would absolutely meet with a doctor concerning this if you are taking prescription medication (particularly if it is something new).
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