> Anyone any ideas why she may be producing milk?
I don't see why you think it's abnormal! :-)
In the wild, all the bitches would make milk and feed the alpha bitches pups. It's a great survival technique.
I have a Yorkshire Terrier bitch that is very, very maternal. Has always wanted to steal other bitches puppies, and has tried her utmost to get mated when I was determined that she was not having a husband.
I wanted to have her spayed last autumn - I always spay midway between seasons. But this bitch had the worst phantom that I have ever seen. She started following 12 weeks old Chi pups around - ones that I had bred, so mum still here, but no longer feeding. My Yorkie girl would pull them under her, and when I found them suckling from her, she was loaded with milk. Unfortunately her milk has lasted 4 months, and when she next comes into season, she will be examined by my vet, and how/when to get her spayed will be discussed.
But your bitch not being pregnant isn't relevant! And yes, I agree that your other girl brought her in to season early. My girls are always doing it.