Mom rented out room in the house to a guy. Am I overreacting?

I'm 22 years old and for the past couple of months, I moved back to my mother's house after graduating college. I moved back mainly to save money but also to be close to my mother before I move out of state.

Ever since my brother moved out with his girlfriend, she started complaining about money… All she wants to talk about is bills and how she can't afford living in this house and how she has to pay for everything. I tried to be understanding, so of course I pay her the phone bill and help out with the mortgage. But at this point, I'm starting to feel more like just a roommate to her than her daughter, and it has been stressing me out.
About 2 weeks ago, she mentioned to me that she was thinking of renting a room to this guy we are acquainted with. I told her NO that would be awkward and it's a bad idea. Last night, I get a call from her saying "The room is all cleaned up and it's being rented out!" She was all happy. I literally felt sick to my stomach. How can she just call me when I'm on my way home, telling me some guy is living in a room across from mine? And the fact that he's a GUY?

When I got to my room, a mattress was missing from my bed (two mattresses make my bed taller) And she said she gave it to him. So I guess he doesn't have a bed? I'm so uncomfortable in my own home! Am I overreacting? She keeps victimizing herself, saying she needs help financially even though I was helping her. I told her I want to move out and she still wants me to stay.

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