If this is only when you bend over and NO other symptoms develop,then you are experiencing normal neuro -vascular reaction. It is due to the change (reduction)in blood flow to the brain at the instance of the bend (probably around the vena cava- located in the abdomen) and the addition of symptoms could occur the longer you remain in the position.The addition of symptoms could also mean you have been more seriously deprived of oxygen and should seek medical attention.
I hope this helps.