Hi, It sounds like it could be a couple of things.
(1) Every flock has a pecking order. If no males are around a hen will take over.
(2) She is broody. Not laying eggs, protective of eggs and or chicks. Always mad looking puffed out.
(3) She is just plain mean or Bipolar It happens.
Chickens as they grow can make some funny sounds till they get their cluck or crow down pat. All roos sound awful when first starting to learn to crow. Some sound as if they are in pain.
Sexing can be tricky till the are around 8 weeks old. Roos will stand strait, tail up, wings tight to their body, many EE's will have red patches on the wings, with red combs and start getting pointed feathers around the neck and base of tail.
Hens are softer looking. Tail is droopy, wings relaxed, comb is pale or light pink. color is uniformed and not patchy. They start to pair up at that age. Little pullets start to hang out with 1 favorite roo. Right now my 10 weeks old have a favorite roo. He has 5 pullets hanging around him and the other 4 roos run together.
I have never seen a fat leghorn. they are mostly skittish and lean. I did have a cornish X my kids got as an Easter gift from a friend who worked at Koch farms. It was a gross chicken. solid white and just sat in front of the feed bowl. It didn't move much and got fat quick. It just sat eat and pooped where it was. By 10 weeks old I dressed it out and it was 11 pounds! We called her Chickenzilla. i will never get one again.