I have a neighbor that is annoying beyond words. What type of intellectual disability is this?

I had an idea of this in my head. Forrest Gump (which I know is a movie) and people with Down Syndrome. Which I have another neighbor who has Down Syndrome he's an older man who goes to work 5 days a week from 6am-5pm. He isn't nosy and he's sweet as pie but also cautious.

But OMG this girl on the other hand seems to be normal (she speaks well) but she dresses like Pat from SNL http://en.wikipedia.org/wiki/Pat_%28Saturday_Night... (It's not quite this bad but it's pretty darn close) but she's about 18 to 21. When she first moved here she knocked on the door every time we were home asking random dumb questions: Will I be safe from the storm? Did your lights go out while I was gone during the storm? I was so annoyed and my boyfriend explained to me that she's "retarded". I'm like no she isn't she's just a weirdo. I'm still not sure because I had this idea of mental retardation in my head (Forrest Gump, people with Down Syndrome). But maybe she is I'm just trying to figure out what type it is. She's not sweet, she curses people out on a regular bases, she talks crap about everyone in this building where we can hear her. She has one neighbor that will deal with her so on a regular basis she waits for her to get home and talk about everyone in else in the building in the hallway...some examples are: One guy wouldn't let her borrow his can opener, she told her only buddy that he needs Jesus. Another example she said she smelled something frying and it must be another guy because he's BIG and then laughed like a banshee but she's at least 250 if not more and a shapeless blob.

I believe she thinks she's normal but the only evidence that I have that she isn't is two vans that pick her up: FAMMAT, LLC (a local non-profit organization that I believe deals with mentally disabled people) and the local Community Services Board. (also known for dealing with people with people who are mentally handicapped in one way or another)

I never knew someone who's mentally disabled could be this mean, catty and bratty. I just had NO clue I'm completely ignorant to this type of retardation. And if you know what type it is please tell me.

I don't know why her family let her live by herself. She doesn't seem to have any purpose she goes to FAMMAT and Community Services Board and goes to work maybe 2 a week, and she's a nuisance. We can't even have any quiet time anymore because when she's home she's constantly on the phone and she's so loud we can hear almost every word of her conversation. Her feet hit the floor in the morning and she's already on the phone.

She obviously wants attention because she's constantly in everyone's face about all types of random crap. So why didn't here family let her live in a group home. Because she's is bugging the crap out of all of us. We tried ignoring her for the first 2 weeks and be more understanding because we think she has a problem but she's such a brat. Again I never knew someone with these problems could be this annoying and rude. I'm just confused and this is something new and I would like someone who has a child like this or who has dealt with these types of people in the past.