
Bitches can be extremely difficult, but even male dogs do at times fall out for no obvious reason. My oldest Golden Retriever (13) and my Cavalier (14) have now been kept apart 100 % of the time for the past 11 years! The Cavalier was 3 and the Golden 2 when the Golden started attacking the Cavalier. (The Cavalier NEVER fought back.) We neutered the Cavalier to increase the distance between their status, it made no difference. So we've just coped and kept them apart. It's just so dangerous when a much bigger dog goes for a smaller one. We had a behaviourist in and we muzzled the Golden but even when muzzled he went for the Cavalier.
Having said all of this doom and gloom, I certainly agree with others that the first thing to do is check the Labrador isn't ill in any way and yes, if she is coming into season (or even worse, about to have a phantom) that can make a huge difference.