My mother (55 yrs) felt dizzy and faint-like after eating cereal. She didn't get much sleep but her blood pressure is normal, what could this be?

Too vague. A new sensation that you are about to lose consciousness is relatively serious in a 55 year old and needs explanation. If she were dehydrated and stood up quickly that may be enough to explain her symptoms but if not she should see her doc to see if there is something reassuring in her story.