My supposely best friend back stabbed me and my other friend??? WTF?

Ok so, I had this friend and she got kicked out of her house but she's only 17 (her parents are crazy) so, she would live with me sometimes and with my other friend sometimes. One day she was at my other friends house and my friend had to go to work and clearly told Amanda NOT to have anyone over. Reba comes home from work and finds Amanda and this guy Brent in her room finding out they had sex in there on Reba's bed and Reba was NOT very happy. (I don't blame her) amandas crazy dad made her go home or he was going to call the cops on her and she went home a day latter her dad comes over saying that Reba was home and knew Amanda and Brent had sex when reba was indeed at work and she said I'm the on that gave her the hickey (used as a cover up) and I also found out she's said some really mean things about me and she's told me really mean hinge about reba. After 4 years of being friends why do you think she totally back stabbed us???? For everything we've done for her. ^_^