This probably results from changes in blood volume and blood pressure regulation by your body and may leave you feeling weak and dizzy. It may also be because you''re not eating enough and your blood sugar level is low (very common when you are suffering from sickness and or nausea). If you do feel suddenly faint and dizzy, try to sit down, have a sugary drink and eat something. Try not to go for long periods without food. Little and often is the rule. Carry biscuits, fruit or sweets with you when out and about.