Well if she was at least 4 years old, it is less likely that it was Marek's but I wouldn't totally rule it out, especially as you have just had her a month. I'm shocked that anyone would "sell" 4 yr old hens.... they should have been looking to give them away to a good home, especially if there was a rooster with them! You have obviously been taken advantage of with them. Do you have other chickens of your own as well as the ones you got from her? Is the rooster mating them too?