I'm 21 weeks, second pregnancy. When I walk a lot or am cleaning or holding my 2 year old my stomach gets really tight. I can feel it starting and usually can stop whatever I'm doing and sit down and it'll go away. It literally feels like I'm a balloon and being filled to the brink of popping. It only happens when I'm being active tho so I wasn't sure if it was like just muscles getting tight or Braxton hicks or what. Never experienced this with my first. Any insight?