Tightening in belly

beeee • Daisie Annalise born on her due date 12.03.17 at 9:26 21 inches 8lbs 9oz💕

So ever since about 13 weeks, my belly will be tight and pop out more than usual when I'm laying on my back it only happens sometimes . I asked my midwife about it and she doesn't seem concerned at all but it kinda worries me

Can someone explain what this is and if it is normal?