I think it's a 3rd trimester thing. This isn't the first post I've seen about this. I've been on & off with the same issue, along with being constipated in between.
I started the same problem at 35 weeks. Don’t get your hopes up. I’m going on 37 and still nothing. Watch for other clues like contractions and bloody show/losing your mucous plug. I think it’s definitely our bodies getting ready for labor but it could be days or weeks before the little ones are here.