I've had this over the years but rarely. Happens when I lay on my stomach or stretch and doesn't last very long. But it feels terrible like string string from my navel to my to my bladder is being pulled or injured. The first time it happened I was pretty worried. I see many other posts on here and other people seem to be worried. So I believe it's just strain on the medial umbilical ligament. If there's no diagnosis don't worry. Hope this helps.