What causes sinus tachycardia, dizziness, and rhythm changes when standing, after eating a meal, and after waking up? ekg and echocardiogram normal

Hypovolemia: Most likely relative dehydration to some degree. There is enhanced blood flow to the gut after a meal. This means the same blood volume has to supply more to the gut leaving slightly less elsewhere. This can exacerbate borderline dehydration leading to compensatory higher heart rate and occasional symptoms. Try drinking more water as an initial step especially in the summer heat.