When i stand up,if i hold my breath while standang up i go a bit dizzy.everything goe dark and sometimes i fal?

i dont remeber the fall..but when im on the place iv fallen im aware of stuff but i dont know whats going on im just worrying someone is gonna walk in on me on the ground..kinda hard to explain but thats about it...iv lost nearly 1 and a half stone in the past few months but iv put on a few pounds over xmas..i do eat ..a bit less then i used to a few months ago but still do eat ..my bmi is baderng between normal and underweight..sorry this is so long please helpxx