My one ex was like that. Difference is she was dependant on me. I started getting suspicious when she started locking her phone and bbming excessively. Hanging out with one if my friends a lot. He said something to me the one day that was supposed to be disarming and I found it very suspicious.
So I watched her over time and figured out her phone pin. And found some she had been cheating on me with one of my friends. Hacked into her fb account, apparently her friends knew too. I was so far beyond hurt.
Well that was the end of that. I took a bat to his car and beat the crap out of him.
She was the sweetest kindest girl I'd ever met, she was sexy and fun. I as madly in love. But what I realized was that you can never truly know someone. She was evil in a self possessed way, the validation she got from two men fueled her self image. I never even thought her capable of it. I was living together with her aswell.