When you say it happens when standing, do you mean when you stand up or it only happens when you're already standing up?
If it's happening when you stand up, I would guess that it's the baby dropping lower when the space opens up (sitting pinches that space a little bit) and causing little contractions as the ligaments (the aponeurosis of the external obliques, technically) don't stretch much and your uterus is getting pushed back when it pushes against it.
I get those contractions frequently, but they're not too painful for me, probably because this is my 5th and everything is stretched already. Bring them up with your doctor when you see him/her or if they get really bad or frequent, go in to L&D, but those contractions are probably have no effect on the baby or your cervix.
