come in heat
come in(to) heat
To be in a state of sexual excitement and able to breed, as of a female animal. Do you hear the stray cat howling and screeching at night? She must have come into heat.
come in(to) heatand come in(to) season
[for a female animal] to enter into the breeding season. This animal will come into heat in the spring. When did your dog come in season?