Stomach contracts on back

Hi y’all. I am about 21 weeks with twins. I have had no Braxton Hicks or anything BUT if I lay on my back, my stomach instantly contracts until I roll onto my side. It’s the strangest thing. This didn’t happen with my last pregnancy. (I know it’s not Braxton Hicks. I had them every day for about 4 months with my last pregnancy.) Anyone else experience this?