Woke up in the middle of the night from heartburn, then started feelin…

Woke up in the middle of the night from heartburn, then started feeling brief pain in my right lower back. It's not even pain, but an unpleasant pulling sensation. It grabs and lets go for a second. The belly doesn't hurt, it's not hard. What could this be? Like contractions, it doesn't seem like that. Although I don't know how contractions happen, I have my first planned C-section scheduled.