Hi there,
I get this as well, think I even posted about it a month or more ago.
I think someone responded and said it has something to do with the ligaments stretching, but I may be wrong.
I get it when I cough or sneeze and try to tuck my knees in when it happens.
Hmmm i'd be curious to know what causes it as i'm only 14 weeks and have had it for a while.