In all seriousness, it could be a lot of things. Low blood pressure, hypovolemia (low blood volume), dehydration, diabetes, anemia, or even Addison's Disease, a rather rare disease where your adrenal glands don't produce enough of certain hormones, for example.
DO NOT IGNORE IT. Trust me.
I had that for a while, and it got so bad in me that once when standing up I passed out for about a second, whereupon I decided to go get checked out. Turns out I was severely anemic, so much I shouldn't really have been conscious except it had been happening so slowly my body had gotten used to it. My hemoglobin was a third of the minimum it should have been. Ended up I have ulcerative colitis and was slowly bleeding into my gut, slow enough I didn't notice, but enough that it got to that level. I needed a transfusion, 4 units packed red cells.