What is this girl doing and what should I do, if anything? Should I say something to the guy sort of involved?

This is actually probably a question that girls could answer better, but guys are definitely allowed to put in some input, especially as to whether they think the guy has noticed anything or not.

There is a girl in my Arabic class who is really kind of loud and at least during the first few days she really wanted to be the center of attention. She would flirt with a bunch of the guys in the class and still does flirt with some. She's tall, thin, and has a Russian accent and she's not unattractive (I don't ever like to call girls ugly or even say that they aren't pretty, because I think all people have attractive and unattractive qualities, but because of how she has made me feel, it makes her unattractive qualities stand out) but I don't see her as being really anything special. She's a flirt and she's loud, which is why she gets attention.

There is a guy in my class (who I asked a question about before) who I'm pretty sure likes me. Some days are more "intense" than others, but I think the interest is there.

I think one day this girl saw him and I talking and having a laugh and maybe she also picked up that maybe one of us or both of us has an interest in the other.

So sometime a few weeks ago I was sitting in the lobby before class just waiting until I could go to the class room, also waiting for other people to show up to talk to. I hear this girls loud laugh coming down the hallway and she is just absolutely cracking up about something, and the guy was kind of chuckling. They both looked at me when they entered the lobby and she sat down right next to me and he sat across from me. It felt kind of tense and its hard to tell from guys inexpressive faces sometimes, but the rest of the night was kind of uncomfortable between us, even in class when we were talking.

The next class, the guy showed up much earlier than he usually does (I show up early too) so we spoke. And the next time he was early too.

A few classes later she kept asking him questions about things, leaning into him and laughing... him just being very casual.

Then today again I heard the laughing coming from her and saw the two of them walking down the hallway, she was asking him questions about the homework. She sat down somewhere and he sat in a chair near me.

The whole class tonight I would see her turning around to talk to the guy behind her but I could feel her eyes on me as I was sitting next to the guy, right behind the one she was talking to.

She never smiles at me. The first day of class I actually felt like she gave me kind of a sour look after looking me up and down. ONE time I did smile at her in front of other people, kind of forcing her to at least give me the half smile that she did.

I don't get it. I'm a quiet girl. My friends think she is jealous, but I don't know. I don't know what to do. Did this guy catch onto it which is why he seems to keep his distance from her a bit? What is she doing? What should I do?
What is this girl doing and what should I do, if anything? Should I say something to the guy sort of involved?
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