TMI warning

My fiancé and I were doing the deed after what seems like forever (36 weeks preggo) and I was on top since we’re running out of positions😂 after awhile I started getting period like cramps under my belly that ran straight across. Stopped me in my tracks for a few minutes. Now afterwards I’m still crampy with a little lower back pain. I figured I just used muscles I hadn’t in awhile. Could this possibly put me into labor? Or is my uterus just acting up? Any thoughts?