The same thing happens when they get too much soy in their diet -- and all packaged products seem to have some soy in them ---
Turned out that the hair product her mother used in the girl's hair contained mare's placenta. The poor kid was being hit with pregnancy hormones.
Story 2: On my mom's farm her male animals all began to behave strangely. The billy goats had no interest in the girls, the male cats insisted on invading the nests and trying to nurse the kittens. Heck, four week old female goats went into heat. Turned out her grain was infested with a mold that put out estrogen. Every animal was turning female because of mold. (I have to note here, the boy kitties who were born during that time had wonderful fashion sense...) Seriously, the boy kitties who were born during that time never mounted a female. As a matter of fact, we had a problem with the toms "raping" these particular males. For five years, that never changed.
Her vet was fascinated by the whole thing.