It sounds like your rat is being barbered by a cagemate. If this is the case, it's not harmful (though it does look silly!). Does the cagemate have thinner fur on her own forearms? Often rats that barber their cagemate's head will also shave their own arms. It's hard to say why some rats do this, but if you have a bald rattie with no other symptoms (and a guilty looking cagemate), this is often the cause.