Blood pressure 140/104 then normal reading 136/89 on second time, why?

Usually no reason: Most people who check their blood pressure readings don't get the same reading on repeated measurements. Humans are not identical from moment to moment (we're not robots). A person who finds variety in her measurements can just take 2 measurements every time and average them. She can show the numbers to her primary care doctor for advice. A person in her 20's should have nice, low pressures.