Weird tearing/pulling away feeling around belly button 20-21 weeks

Hey everyone,
A few of times I've had this weird tearing feeling in one area - at the top of my belly button. It only lasts like 1 second and the pain only lasts for just as long.
I can feel that the top of my uterus is around that area, and I've been told my placenta is 'out front' (I can't remember if that is posterior or anterior.)
This is my second child, although my first is now 9, so it's been awhile..
I've not had any other symptom or problem, but am wondering if its normal or if I should go to my dr.?
(My first child stopped growing and I lost all amniotic fluid by the end of the pregnancy- but she is completely healthy etc - but apparently what can cause those symptoms is a split/tear in the placenta.. Hence why I'm slightly worried, but I just want to know if this tearing/stretching/pulling away happens in pregnancy...

Thanks guys.

Could it be muscle separation? I've never had it, so don't know first hand.

I'm not really sure.. I'll ask my midwife when I see her next.