How old is she? It might be that she's at that stage where they are holding in wee then doing one big wee and flooding the nappy (so, like you say, possibly close to being ready for PT'ing).
What nappies are you using? If pockets and/or AIOs then perhaps they are not enough. Fitteds suitable for heavy wetters are the best bet (and will no doubt need to be boosted).
As a last resort, do the nappies need stripping?