What Causes Fainting While Suffering From Heart Palpitations?

HAllo I take beta blockers forpalpitations which occur as soon as i wake up and move very slightly I lay very still and gradually after a few minutes they subside .when i stand i feel very shaky and generally quite poorly my legs feel very weak and shakey.gradually i feel slightly better as the morning progresses i find a hot shower helps.I have experienced a couple of faint like collapses in the past couple of months dont loose consciousness but my legs buckle and i go down.Would appreciate your input thankyou.I am a 64 year lady.