Sometimes we just get depression for no reason. It can be due to continual anxiety, or it may be chemical. Kitt and Rachel have given you some great advice..I would talk it over with a health professional. Maybe all you need are some lifestyle changes or it may even be diet. Who knows?! I can reccomend a great website "beyondblue.com" it talks about different types of depression, causes, medications and all the rest you might be curious about. Good luck and hang in there.
Panic Attack Survivor
Weekend Warrior Princess