2 things here.
1: You're looking too much into it, so she decided to go home so what, she can go home if she wants to. I doubt she thought it'd upset you because, well you were sitting at home anyway right? Also if she was at a party, drunk, or just worn out, she just wanted to go home.
2: Why did you even agree to this, its such a beta thing, being used like that. Unless you only did it hoping for sex, if you did thats sad and if you want to think wrongly of her she used this expectation to ensure you'd stay up for her so she can just fall asleep. Or you're just a nice guy... which doesn't pay off in this situation eitherway. She asked you, it would put you out, so just say no.
Actually, a 3rd thing, how long is "a few weeks" because why weren't you at the party with her? So you've been going out a few weeks, seemingly not long enough to be called your gf. So she isn't your girlfriend but she can just say she wants to sleep at your place and you just roll over and let her.
Honestly I could analysis this for ages and none of it falls on her.
What the fuck do people get off thinking they can treat each other like this?
What the fuck are you smoking, she went back to her own home after being at a party, oh no! What a cunt, AMIRITE. If you made this post on the night straight after then chill the fuck out. She probably went out, got too drunk, or didn't feel super, so went home. Wait until tomorrow and just casually ask "Hey how come you went home last night, you ok?", maybe she'll say sorry etc, but chill.
This is a prime example of over thinking, and sitting there seething over something without knowing anything.
Oh another thing, if you honestly think she was just a bitch, and that is your go to assumption about her:
OR