How does her bitch pee/poo with a nappy on?
Are you aware that putting pants on bitches in-season may increase the risk of infection?
Unless these pants form some sort of cast iron chastity belt, they will not prevent a keen dog from doing the deed, if she is willing...
Pyometra is an infection of the womb - an open pyo can potentially be cured with antibiotics (though you'd still be wise to spay the bitch once sorted), a closed pyo can be fatal within a few days for a variety of reasons (ruptured uterus, sepsis) I wouldn't get hung up on kidney failure.
Berating the OP about her bitch not yet being spayed when at the minute, it would be really unwise to spay (again, spay 3 months after the start of the season to catch her mid cycle when hormones are 'normal', you'd only spay mid season in a dire emergency!) is not useful, and pretty unkind.
There are good reasons for not spaying until a bitch is mature, and it is possible to have an entire bitch and still be a responsible owner, which it sounds as though the OP is.