Constipated or Contractions??

The past 2 days I have been feeling some pressure in my butt/lower back area every once in awhile throughout the day. It doesnt last long maybe a few minutes than it goes away. I don't know if im constipated or experiencing bh. I just bought stool softeners in case I am. I'm not having trouble pooping. I have pooped twice today with no issue. I'm almost 31 weeks. I didn't experience this feeling with my 1st. I see my doctor tomorrow and will be sure to mention this to him. Any thoughts?