I told someone yesterday I would get them surprise. Today I totally forgot about getting and giving gift. Reminded by person. Early onset dementia?

Unlikely. Stress, overload, sleep deprivation, anxiety are a few of many reasons we may "normally" forget something that causes worry or embarrassment. Ask a trusted friend or family member if they think you're more forgetful. If memory problems persist to the point of disrupting your work, relationships or overall functioning or you'd like more reassurance, see your pcp for an evaluation. Hope this helps.