I've got three pet hens who are 3 1/2 years old and have always gotten along with each other. This morning, the dominant hen attacked the lowest ranking one, who apparently decided to assert her authority and didn't give in. I broke up the fight and watched what happened, and they fought again. Only the dominant hen had a couple of scratches around her wattles, but I separated them. Now the lowest ranking hen is above the former middle ranking hen. This is so weird, I'm worried there might be a medical problem behind her change in behavior, but everything's fine from what I could tell, and they're both eating/drinking/moving good/etc. The only change recently in their care is I put a heat lamp in the coop, but I've done this every winter with no problems before. I'm really, really worried about my girls. Does anyone have an idea about what's going on?