Pyometra is a condition affecting unspayed bitches (and less commonly cats) where the womb, or uterus, becomes infected. In mild cases it can come on fairly slowly with only slight changes in the uterus, but the worst cases happen very quickly and the womb becomes swollen like a balloon, but filled with pus. These are urgent and life-threatening.
Pyometra happens when the lining of the uterus (the endometrium) changes under the influence of the bitch’s hormonal cycle. It nearly always happens a few weeks after she has been in season and is more common in older bitches.