Why would she say such horrible things to me after a 1.5 year relationship?

My girlfriend and I were together for 1.5 years, and then one day while I was out, she snooped in my phone and found a number from an old friend, another girl, let's call her Alice. I hadn't talked to Alice for 2 years, but she sent me a text message recently saying she was in town, and wanted to meet. I casually said sure, let's meet up sometime. We never actually met, nor did I intend to. Anyway, my girlfriend sees this exchange, and immediately assumes I'm cheating. She didn't bother to ask me what's going on, she just left, blocked my from facebook, and blocked my number from my phone. I had no idea what happened, I just came home to find that she disappeared from my life. Anyway, I've been trying to figure out what happened. Apparently she called Alice, who for some reason told my girlfriend that if she can't trust me she's in a bad relationship and should leave. So that's what she did. I tried visiting her and talking to her, but she started making threats and screaming at me to get away from her, so I left. 3 weeks later, I give her a call to find out what's going on. I just wanted to find out what can possibly happen to a person to make them go from love to hate in just 1 hour. She started throwing out abuse, saying for example, that I could never please her as a man. I reminded her that a month ago she said I was the best boyfriend in the world, to which she replied that she only said that because I had low self esteem and she wanted to be nice to me. But none of this makes sense. We were together for 1.5 years, so if she wasn't satisfied she should have left. It seems like she's just saying whatever she can say to try to hurt me. I don't care if she wants to see me or not, that's her choice, but I can't understand why she's being rude and kicking below the belt all of a sudden. Why would she say things like this?

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