I've read that it could be in the water, which would also make sense. And considering the zombies consistently getting stuck in the mud and the one in the well, there's at least some tangible connection.
Considering all that seems like it will happen at the barn next week, I can't imagine a major character being introduced. I don't know when they meet her in the books, but it would have to be after they leave the farm I would think, otherwise it means she's going to rush in and save the day...which means it was a huge coincidence or she was just living on her own at the farm anyways.
I just don't think the episode can cover enough ground to kill all those walkers, make a decision on where to go, leave the farm, and run into another major character.