My 9 week old is nearly 13 pounds. She recently went onto size 3 nappies, they were fine for a couple of days but then she's started leaking at the back? Normally we would go the next size up when she started to leak but these don't seem that small on her?? & we are putting the nappy on the same way as we usually do.... has this happened to anybody else? i have used Asda and Aldi, both doing the same!!