If you don't want to read the backstory, just scroll down to the part that is after the stars...

As you all know, I am having roommate issues. Big problems actually.
My roommate was my best friend for 5 years. Like, the best friend I have ever had in my whole life and she always said the same about me. And suddenly she stops talking to me and won't talk to me about it, and when I finally get her to talk and ask her why she won't, she said it wouldn't matter; nothing was going to change.
To me, she basically said I didn't matter. That our friendship didn't matter. That her boyfriend, who she literally spent every moment that she possibly could with him. I literally saw her 5 times, maybe in the course of 30 days. She literally slept in her own bed by herself once. Which fine, whatever. I have the whole apartment to myself then and I'm only paying half the rent.
Now, let me just say that from the beginning-even though my name is not on the lease, she has always told me that this apartment was half mine and everything in it was half mine. It was our apartment.
So, I come home one day and she tells me her boyfriend's moved in- for a couple of weeks to find a new place. Now, since I have been looking for a new place, I know there are several affordable, available places in the area he wants. He doesn't have a car, so my roommate has to drive him everywhere. And we live in a town outside the city, so it's not like it's this 5 minute trip.
I had literally zero say, zero warning about this. And I feel that I should have been told he was going to be living there.
But whatever. That's in the past.

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What I need your advice/opinion about is this:
He's been living with us for over a month now. No word about him moving out any time soon. Now, he is home more often than I am. So he uses more of the utilities than I do. He also eats my food without replenishing it, which is annoying when I have sorted out my allotment for the week and he eats it so it's gone by Wednesday and I have to starve for lunch for the next couple days or waste more money to get a lunch.
So, now that it's the end of the month, I told her I would only pay a third of the rent. I don't feel it's fair for me to pay half of the rent because now there are 3 people. And especially since she didn't talk to me about him moving in, there was no discussion about how we would dole out the expenses, who would pay for what. But since he is home more than me, uses more things than me, takes me food and all that, I don't find it fair to be paying for all that. If she wants to fine, but he'd be the one to be able to afford it most! (He doesn't have a car so he has no gas, no car payments, and right now no rent. He has money).
And now my roommate is pissed at me, thinks I'm being unfair because she never made me pay full rent for the time that she basically lived at her boyfriend's place.
But the difference is, she chose to do that. I didn't choose to be in the position that I'm in. I had no say in the matter. SO I think she's being unfair.
But I wanted to know if you guys think I'm being unreasonable. Is it just me?