Right lower back pain/buttocks

This morning I woke up with pain in my right lower back and buttocks.  Nothing serious just like a dull pain.  Later in the day I started getting contractions and they were about 15 minutes a part.  This went on for a couple hours.  I made sure to drink a ton of water and got off my feet when I could.  Eventually they went away and so did my back/butt pain.. Anybody else experience this.? Freaked me out because my second child was born at 32 weeks.  Took progesterone with #3 and currently 2 shots behind on this pregnancy. (#4) Eek! Left a message with my dr letting her know I missed two of my shots.  Hopefully I was just having an off day and it won't happen again..