Is it normal to get heartburn after eating pop tarts and drinking a glass of water at night?

Some basics. Heartburn is the irritation you get when stomach acid back washes into the end of your swallow tube. If you fill your stomach with anything and lay down within the hour, you remove the gravity assist to digestion by laying flat. The stomach churns to process food & the pressure of a squeezing stomach can trigger some backwash. To avoid it, eat earlier/lay down later.
Pop Tarts? Normal no, common yes. You had gastroesophogeal reflux, a very common condition. Tonight, take Tums (calcium carbonate) or some other antacid. Consider medical visit if it continues or recurs.