My room mate

4 months ago I took a room mate, I own a house with a basement suite and she moved in downstairs. She is a decent looking girl, 20, skinny and flat chested but a pretty face, When I say flat chested I mean flat chested, She has barely even a bump on her chest and dark brown ** the size of a quarter.
We hooked up one time, We met in the bar and shared a cab, When we got home she gave me a ** and then nothing was said after about it, She sometimes comes upstairs and watches tv with me. We have a good relationship so far, She pays her rent and we get along when we are together which is not a lot but maybe 5 hours a week or so.
Last night we met in the bar again and I had thought about sharing a cab again but she kind of disappeared, I got home and turned on a movie to pass out on the couch to, A few minutes in my phone dinged and I picked it up, It was a text from her saying "Wanna come down?", Obviously I texted back and said "5 min?", She sent a smiley face and I jumped in the shower quick. I scrubbed as fast as I could and tossed on a pair of shorts then tried to act casual as I walked down the stairs.
I got to her bedroom door and opened it expecting to see her maybe in bed or something waiting for me but what I saw blew my mind, I opened the door to my room mate and her friend who is also a nice looking brunette with curly hair and average sized ** with a decent body. Both were sitting up with their backs against her head board and their legs spread crossed over each others. I stopped and stared until my room mate snapped her fingers, I stood at the foot of her bed as they kissed and giggled then my room mate crawled to me and pulled me into bed.
The three of us banged for hours, I finally rolled off the bed with my ** aching from getting off so many times, I lost count but it was a lot. I went and got a glass of water and returned to find the two of them 69'ing and watched for a solid 10 minutes before watching both of them get each other off. I gave them each a drink and my room mate grabbed my limp ** and stroked it a bit looking up at me and said "You done or can you get it up again?", I shoved it in her mouth while watching her friend lay back spread eagle rubbing herself and managed to get it up one more time but couldn't finish, I nailed them both for as long as I could manage before flopping down and all of us passed out together all sweaty.
Since then she has acted totally normal like nothing happened, What the **?