You could just put a terry toweling nappy in her carseat. I was going to suggest that you start TT now. This is what started happening with my DS (although he was in cloth) and not long after he trained. He would hold his wees then when he let go, his nappy would leak and he'd say "mummy wee". His daycare started putting him on the toilet after every nappy change and we tried to do that at home too. When he started doing his wee's in the toilet, we gave him his big boy undies and he clued on pretty quick. Refused to wear nappies again except to bed. After a handful of no 2's in his undies, he figured that part out too. Give it a go. She might be trying to tell you she's ready & you then can save a fortune on her nappies