stomach cramps 31weeks

Kay
FTM here, I will be 31 weeks on Sunday. I am wanting advice on stomach cramp/pain I have been having today. It feels like I have a seat belt or something pushing down across the middle of my stomach. (Only way I know to describe it) It has been going on since about 1:00 today on and off...only other thing I have going on is some lower back pain but that has mostly gone away..and gas. Has anyone else experienced pain across the middle of your stomach as a sign of labor, or have any advice? It gets worse when I sit down and stand up or move around...but I don't think it's ligament pain because of where it is located. Also my stomach has not been getting hard like it does when I have Braxton hicks. Any advice would be helpful...thank you!