Hi Tong
The first thing is - I think you should go and see your doctor if you are feeling dizzy, having blackouts and fainting. There could be all sorts of reasons for this and you don't want to second guess what these could be.
As CLKD says you could have low blood pressure and also the sudden drop in pressure when you stand (I get this on standing up quite often although I don't black-out and I have low blood pressure.
Are you taking any medication which could cause this eg any which lower blood pressure like beta-blockers for instance?
The fact that you considered menopause means you may have other symptoms as nearly50 says - perhaps your periods are changing and maybe you are missing a few?
If you are heading towards this stage then the general advice is to make sure you eat a healthy and varied diet with lots of fresh ingredients - cut out sugary foods and drinks and reduce the amount of refined carbs ( eg white bread, cakes and pastries) in favour of fruit, veg, nuts, beans, whole grains, low fat protein, fish, dairy food etc. Eat regularly too. Also reduce alcohol consumption and give up smoking if necessary, keep within healthy weight limits, take more exercise, fresh air, low stress and a good night's sleep - sounds like a lot!!
Hurdity x