Sometimes a drop in the amount of (vigorous, aerobic) exercise a person is getting can set off a drop in neurotransmitters resulting in depression. Chronic mild dehydration can do that as well. I think the longer you are depressed the harder it is to dispel it, so if it continues much longer, even it isn't crippling, you might consider getting some pills for a few months. It's too easy to settle for a diminished quality of life without even realizing that that's what you are doing.