Cat skin has small rough patches

I just combed this off my cat's chest.

Does anyone knows what it is? She has a few small patches like this on her chest and under one of her front legs. The ones under her front leg seems a bit more brownish. Is this serious? Do I need to bring her to a vet?

Her behaviour, appetite and everything else is normal and she is an indoor cat and slightly less than 3 yrs old.

Thanks!

Regards,
Jia

Yes, I would contact your vet.  Skin should not be coming off when you brush her.