Hi there - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with cats and dogs. I'm sorry to hear that Tommy has been losing his hair! I'm here to help :)
From what you are describing, the first thought that comes to my mind is fleas. Some kitties are really sensitive to fleas running around on their skin and it will cause them to become easily distracted and lick, scratch, or twitch. There are a good amount of cats who are VERY allergic to fleas and thus will lose their hair and continue to scratch even after the fleas are gone because their allergic reaction is severe. I've also seen plenty of cats with fleas who never scratch in the presence of their owner.
Fleas can hide REALLY well in your kitty's fur, so I never count them out until I've done a thorough comb through with a flea comb. Flea combs are bristles so close together that if you brush over a flea, it will get caught in the comb and you will see it. They are really inexpensive and can be found at a pet store. The best place to find fleas on a kitty is around the rump area and around the neck. Sometimes the kitties are so good at grooming themselves that you won't find actualy fleas on them because they've been catching them and eating them. Even if none of the other cats around have fleas, it still could be fleas causing one cat to lose hair because she's allergic.
I would start by ruling out fleas. If fleas aren't the case, then it sounds like a flare up of some other type of allergy. Usually, when a cat loses hair, it's flea allergy, however other possibilities are allergies to food and things in the environment like pollens, dusts, etc. My favorite foods to try for allergic cats are the Natural Ba***** *****mited Ingredient Diets or Nature's Variety Instinct, although if those don't help we sometimes need to do prescription foods like Hill's z/d or Royal Canin Hypoallergenic Select Protein.
My favorite flea treatment product is the Seresto collar - made by Bayer. It's my favorite because it works and because it's the cheapest option out there for long term control. The collar may look expensive when you shop, but keep in mind that it will absolutely eliminate your flea problem and it lasts for EIGHT months. You can't buy 8 months of any other high quality flea product for that price. If it's not fleas or if the fleas aren't totally controlled, she will continue to itch. In that case, many veterinarians would administer steroids to help calm down the allergic reaction. Usually there's not a whole lot of testing required, just a good thorough physical exam. If a pet doesn't need steroids frequently, many vets just treat the symptoms as they pop up - we don't always dive straight in to a lengthy food trial or allergy work up. We would do that if the signs are unrelenting or severe or not really responsive to treatment.
Please let me know what other questions I can handle for you :)