Is this normal for a 6 y/o?

Didn't know where else to post this but hopefully someone has some insight...So my little sister just turned 6 last August. However lately she's been off in her own world more frequently than ever before. She's the youngest of all of us (next youngest is 17) so it's almost like she's an only child and she plays alone most of the time. She's also very anti social when there's a lot of kids around but if it's just one or two she's a little better and eventually starts playing with them. However, when alone she talks to herself a full blown one-sided conversations as if someone is actually talking back to her. And sometimes she'll space out while playing but she doesn't realize what she's doing (for example, we went to a restaurant a couple days ago and she was sitting there quietly, off in her own world and somehow she ended up under the table) she can't sit still at all. And now she'll be doing something alone and we'll call her name several times and it's like she can't hear us until we yell her name and then she suddenly looks up. It's like she's so far into her world that she didn't hear us calling her. Is this normal?