Is NOBODY wrong in the post ? POST : Throwaway:

Back story: My mother came into a lot of money when i was around 12 and didn't do anything with it, when i got older myself and my brother told her to invest this money as its doing nothing sitting in a bank account, she bought an apartment, and living/going to college in the same city, she offered me and my girlfriend to rent the place with a third person. We did this and have had 2 house mates, both fine and moved out for their own reasons. I leave to go to another country on my once a year holiday on Monday, and today (Friday) i get a text asking me when i leave. I told her Monday at whatever time from whatever airport, and she sends me this : 

"I've offered the use of the spare room in [neighborhood] to [name] my colleague in [work] for a few nights. are ye ok with it."

my reply:"When will they be here" 

Mother: "[They] havnt confirmed that [they] will take the offer but next week. he has to finish his dissertation by [date im on holidays]"

my reply: "I mean. if [they] needs it i suppose. But id prefer if you run something like that by me before offering... Im not very comfortable with someone i dont know in my house while im not in the country"

Normally I wouldn't even have to ask AITA, but here's the context , Im unofficially living here, i have no contract with her, and i get a family rate, i pay 400 a month and my gf pays 400, for an apartment that could easily get 1800 if rented to the public. My mother pays for what are called service fees, 1200 a year, they just keep the buildings painted and tidy and pay for the use of communal bins. Shes not saving me a huge amount of money, i would be paying the same somewhere else, but my place definitely would not be as nice. 

I dont mind her friends staying over or people that i know and she knows, and even when she needs to say some times (im in a good location for her for staff/xmas parties or just nights out) But i have no idea who this person is, and not only will i not be in the house, with all my stuff (Xbox, tv high end PC with expensive peripherals - and sentimental stuff  , things from holidays with my girl friend, photos etc just stuff i cant afford to replace for any reason) I wont even be in the same country! 

The thought of someone in my space, with mine and my girlfriends stuff, who i have never met and have no idea what they even look like or act like fills me with anxiety and makes me deeply deeply uncomfortable. 

Am i being an ass for not being okay with this considering that she let me rent here for such a low price for how nice the place and area is?

side note: I am planning on increasing the amount i pay after i get my degree and start working after college in just under a year - I dont believe that i am taking advantage of her, but i guess i would like an outside perspective on this?