For the past couple of weeks occasionally if S has a wee lying down, somehow it will go straight up the back and out of the nappy soaking her layers of clothing. Does this mean the nappy is too tight, too lose, too big or too small? We moved up a size maybe 3-4wks ago and she's now in Pampers 9-15lbs size and she weighs 10lbs.

We very rarely have a blowout and all poo is pretty much contained in the nappy. So it's just the wee! What are we doing wrong?