Overgrooming? Bald spot

I've recently noticed a bald spot on my cat Mama under her shoulder blade. It wasn't there earlier today and now it is a solid quarter sized bald spot. The last few days I've been noticing Otis's fur getting coarse in the exact same spot and his hair has been thinning but he doesn't have the distinct bald spot that Mama does. So far Baby Girl isn't really showing any signs of the same spot. The difference is that I brush Otis and Mama every few days due to my mom's allergys. I've been brushing them outside so I'm worried that they may have gotten bit. I've just recently started brushing them as frequent because I've moved back into my parents house. I've been using an undercoat brush with the ridges (looks like a saw blade). Is it possible that I'm overbrushing them? If so would it be in the same spot in both cats? Is it possible for them to have something like ringworm that would develop in the exact same spot?

Hmm, well my cat would get bald spots in certain places each summer. We took her to vet and she would have to get a cortisone shot and they went away just fine until the next summer. We eventually discovered that her outdoor adventures were causing her to get bit by fleas so we got some advantage and the bald spots never came back. Maybe them being outside is causing some biting, I would check with the vet to be safe before it gets any bigger.

It is possible that they'd both get ringworm in the exact same spot. Not super likely, but definitely possible. I'd recommend a vet trip-- what would you do if you suddenly had a bald spot on your head?
It's also possible that they're allergic to something in your parent's house, sinc eyou've just moved. I doubt you're overbrushing them- that wouldn't result in a uniform bald spot.