Depression just is, for those of us who suffer from it. It really has no rhyme or reason to it, as you can take many people and expose them to the same situation and only depressed people will react to it with depression. It is an illness, something wrong in our thinking with no known biological cause at this point in time. When it's episodic, as it appears in your case, there can be triggers -- what you're calling stress -- but others have the same things going on and don't get the same emotional response. Are you doing anything to try to fix this problem, such as therapy? As for the other feelings, I"d get a check-up with your doctor. Getting faint and dizzy can be anxiety symptoms, but they can also be a sign of nutritional deficiencies, eating disorders, thyroid problems, etc. Best to get checked out before self-diagnosing this part of it as part of your depression.
The thing about being diagnosed with depression is that it's not something they are going to pick up with a diagnostic physical test. Your words and the way you answer things clues them in. Some doctors screen for it and others don't. Good ones screen for it! That's unfortunate that your doctor (s) haven't picked up on it.
Your symptoms definitely can be mentally health related. But you want to make sure you fit the criteria. People can also get periods of the blues or what could be called a funk.
How is your rest? Sleeping okay? Dehydration also causes these types of symptoms. So, you want to make sure you are eating right, sleeping well and staying hydrated.
If you have been to a doctor, what have they said is the cause for how you are feeling if they didn't mention depression (and remember, you are an advocate for yourself, you also need to tell them that you wonder about it).
There does not have to be a trigger for depression.