Oh, and yes, you can have those symptoms and not be depressed. You can just be sad or suffer grief. Life is full of stuff.
There is no known chemical imbalance involved in depression, so if there is one it hasn't been found yet. The theory behind chemical imbalance is that here must be some reason for it, and current research is focusing on glutamate receptors, but it could just as well be a messenger problem of signals as a chemical imbalance. Mostly, the chemical imbalance theory most people think of was spread by Eli Lilly and then other companies when they came up with ssris and then snris. Serotonin wasn't the problem then and it isn't now. Depression can happen to anyone, sometime triggered by an event and sometimes from out of nowhere. Some day we'll know why, but we don't now, and we still have the arguments about genetics, chemical imbalances, and traumatic events as the cause. My guess is different people have different causes.