I'm a student and I live with 5 others in my flat, one of which being the guy in question. He is seeing a girl and she often comes down to stay with him. No problem with that in itself.

My flatmate is a great friend of mine - we spend a lot of time together, cook together, share shopping, etc, and I love this arrangement and I know he does either. 

The issue is that this girl has been staying for an increasingly ridiculous amount of time. The first couple of times she has stayed for reasonable amounts of time that I have no issue with, such as long weekends. However, the last couple of times she came down she stayed for 5 days, which though I wasn't too happy about I let slide as it wasn't too different to her usual 3 or 4, but the next time she stayed for a total of 10 days/9 nights. 

I was livid, and ended up feeling forced out of my own flat for the majority of that time. Most of the reason I was pissed off was that she was eating my food, drinking my drink etc, without offering a penny towards me, or even a simple thanks! Personally I would offer money or at least bring a bottle of wine as a thank you, at the very least say thanks, if I was even staying at someones for the night let alone the next 10 days.

It doesn't help that we don't really get on either; she is jealous of the relationship that my flatmate and I have, and even though there is nothing romantic about it at all, she seems to try her best to keep us apart the whole time she is at our flat. I wouldn't say I was jealous of her but her presence does irk me; I'm a creature of habit and her being there fully disrupts my meal routine as my flatmate and I can't cook for each other as normal. However, I stress that this last point wouldn't bother me if she wasn't so rude to me in my own home. 

Anyway, he didn't tell me about her coming until the day before she did, when I told him that she wasn't welcome for that long in our flat, especially not without asking me or any of my other flatmates about it. An extra person in our already small flat was a lot to handle, and it meant that we had to have predrinks in our bedrooms/eat in our bedrooms as we didn't feel comfortable being a 3rd party to them. He basically told me it was none of my business.

Problem is, I couldn't just let it go - I'm a student so I'm not exactly wealthy, I certainly didn't want to fund somebody else. My flatmate promised me we'd work out something regarding money for the food she'd eaten. 

After the 10 days elapsed, it transpired neither of them were planning on giving me any money. He said they'd done the groceries, but they had only bought at most around 10 pounds worth of veg, most of which they'd eaten. He told me this was fair and couldn't do any more as he was low on cash. My argument is if neither of them could afford the extra money, she shouldn't have come for 10 days.

Am I the asshole?