No need to worry! That 'knobby thing' is the oil duct, and it's a normal part of chicken anatomy. Like all birds, chickens have a special gland near their tails that produces oil to keep their feathers in good condition. If you watch your chickens when they're grooming themselves, you might see them nibble or rub their heads on their oil duct. The fact that it seems redder than usual means it's probably irritated, likely from too much attention from the roosters. It might be a good idea to invest in a hen apron, or at least clip the toenails on the roosters.