Unusual heartburn . Answers please

I haven't really experienced heartburn throughout my pregnancy but I have just woke up at 2 am with terrible heartburn. Does anybody know why as I was told by the midwife that my baby is at the brim meaning a top of my cervix so is dropping into my pelvis and a sign of this is thst you have less heartburn due to the baby being lower. Do you know why I have just started with heartburn all of a sudden although my baby is lower? Answers please. Thanks