It is most definitely a pullet. Her coloring is female specific and she shows no male feathering at all, and she would by that age. There are lots of reasons why the flock may be attacking her. Illness is one. It could also be that she won't allow the rooster to mate with her, and so he's still trying to establish dominance by attacking her. It could just be her passive personality that is setting them off. And one weird possibility is that she may have a damaged ovary, which could explain both the large red comb and why the male is attacking her. A pullet that has lost her ovary will start taking on some male characteristics, and that could set the other males off. It's a very small chance, though. More than likely she simply has a large comb. Comb size can be variable.
So, what to do? I would keep her in a crate in the run for a while and see if that doesn't help after a few days.