Yes, depression can be a result of the body trying to conserve it's energy supplies when it feels the body is under threat. The body systems close down. Also the dopamine system can be involved - reducing the reward feeling. (and the serotonin system)
I tell you this body is a very complex thing.
However, remember it will pass (at some stage) and then may come again and pass again, You do go up and down.
How's your diet going, have you managed to get your heart rate up with some low impact exercise etc. Even walking up and down the stairs a couple of times may help get the heart beating a little faster.