It may be related to a change in the pH of the blood, to the blood pressure falling, to secretion of certain hormones. I don't know, but it should be studied because, as you said, it is important to know the reason. Sometimes topical steroids can work. Make sure that drops in the other eye aren't the problem. But that's rare.
Skinny girls live longer than fat ones.
Yes, I get lightheaded. I don't know why my blood pressure is so low. It always has been low. An internist could possibly help. There are almost none, literally.