If you do too many push-ups in a row (or any other strenuous exercise or activity where your head is lower than your heart) without stopping to let the blood pressure equalize periodically it can cause what is called a subarachnoid hemorrhage which can result in ongoing lightheadedness and persistent cerebrovascular insufficiency. Cerebrovascular insufficiency can cause ongoing symptoms of depression because your brain may not be getting adequate blood sugar. The brain only needs about 30% of the oxygen that goes up to it but it needs as much energy (in the form of blood sugar) as it can get. Generalized cerebrovascular insufficiency resulting from damage to the arteries at the base of the brain is difficult to diagnose and can cause lightheadedness and incoherence– often due to inadequate blood sugar-related problems. If you do a search online for an article called “Depression Symptoms of Circulatory Origin” you might be able to learn more. I’m not saying that this is what happened to you but it is possible– especially if the problem persists for more than a few weeks. Best wishes!