They often only bleed for a few days, some not at all. She could well be pregnant if she was a stray, and it is not unusual for your dog to be interested in her for the whole of her season but once she has gone past the days she is receptive to the male (often around day 10 - 12) she will reject his advances. She could have been mated by more than one male, and could still not have reached the crucial dates, so keep your male away from her. Best time to spay is halfway between seasons, so about three months time, but if she is pregnant the first signs are usually the teats getting firmer and going pink. She will then gradually put on weight but if only a small litter you may not notice. Well done for rescuing her, I hope you don't end up with a very extended family in nine weeks time!