Do you faint if you see, or maybe even think about blood?
The symptoms you are experiencing are part of a syndrome called vasovagal syncope syndrome.
Syncope means fainting, and vasovagal has something to do with dilating blood vessels. See, when you hear the word "blood" enough times, it triggers the part of your brain that controls how your blood vessels narrow and widen. More specifically, your brain orders your body's arterioles, which are basically really small arteries, to widen. This causes a dramatic drop in blood pressure throughout your body. Since your brain is suddenly getting less oxygen-rich blood, you faint.
Blood isn't always the trigger for fianting. For some people with vasovagal syndrome, it could be anything. Good thing is, falling down is enough to get the blood flowing back to the brain, and then you wake up.