Why Can`t My Cat Purrrr HELP!!?

My cat is 11 mo. Old. My cat is a mama`s gyrl. When she`s getting pet, she tries to purr and soon asit starts, its almost like she gags. She`ll look at me and wana get petted somemore. Mind you her blood brother does the same thing. Mind you they NEVER did this as kittens. I allways remember them purring on mommy chest or on mommy leg. Has anyone experienced this in thEir lil oNes. Wana have an idea what to say to the vet. Please help!!!