Its important to know that female ferrets (jills) go into heat every spring. This usually happens at the beginning of spring but you will be able to tell because her vulva will be swollen and she will stay in this condition until she is mated or tricked into mating by a male who has had a vasectomy right up until the end of the breeding season. Being on heat for so long can cause a problem called aplastic anaemia which could possibly kill her so you need to take her to see a vet to get an injection called a Jill Jab instead of having to breed her. If you know that you are never going to breed from her then consider getting the vet to neuter her.