But I would be keeping her indoors as a precaution. Most girlies don't come into heat until at least six months, but I had a girl kitty who did go into heat at five months. Unlike other cats we've had who have been in heat, she showed no signs of being in heat or of any discomfort at being shut up inside. It wasn't until she was mating under my bedroom window one night that I realised that she was on heat!!