My very first breeding female went into heat at 4 months, I felt educated about breeding, but when this occured, I was clueless. How could this young kitten go into heat, the vets even thought I was crazy. She went in and out of heat very frequently, and by the time she was "almost" 10 months old, I went back to our vet, explained the situation fully, and they said to go forth and breed her, as we didn't want to have problems with possible pyo. She had a healthy litter, nursed them all well, they went home at 12-13 weeks, and she again started cycling hard again. At that time, we decided accupressure was the best thing to do, if we wanted to keep her intact for very long. That helped, but she still cycled more frequently that many of our other cats. I started doing research about a pill, that helps to delay ovulation, but the vets were not too familiar with it, and said it would cause more problems. She was bred again 8 months later, again healthy babies, but by her second pregnancy, she was very thin and wore down. She didn't look like the same cat, even though she was on a premium diet. She did keep cycling, but now by age 19-20 months old, it had slowed down to what I felt was more normal. We waited a full year for her last litter, and again this beautiful large girl, was drained again of everything she had nursing the babies, so she was spayed after the 3rd litter. She was almost 3 years old. We had hoped for her to stay with us, after spaying, but she was a Momma who loved to have all the attention, so we made the decision to let her go retired spayed free with her little neutered baby boy. I know I had made the right decision, after I started receiving pictures of her and her baby, along with emails about how wonderful they had fit in the family.
Nial, I hope you have an easier time with your girl. I believe in most cases a female should be 12 months+, and wait almost a year before being bred again, however, in some cases, I think nature feels differently. Keep a close eye on her, and hopefully, she will wait awhile before going back into heat. Accupressure was the most helpful in this situation. Good luck!!