My girlfriend of 1 year 3 months went out with sister clubbing and held hands flirted and exchanged numbers with another guy and tried to cover it up?

Okay so, me and my girlfriend live together at my parents house. She recently moved in with me to try and save up for a mortgage deposit. Absolutely everything has been perfect up until now. Saturday night / Sunday morning my girlfriend came back from a night out clubbing with her sister and confessed to me she had been crying that night. (But this was not unusual for my girlfriend to cry after heavy drinking) so thought nothing of it and cuddled her and said everything will be ok. The following morning /afternoon when she woke up she confessed she had been dancing & holding hands with another guy but had told him she already had a boyfriend and wasn't interested in that way. I asked if she thought he was cute? And she said yeah. I then asked if anything else had happened that night as dancing with another person isn't a crime. I also asked if he asked for her number in which she replied that nothing else had happened and he did ask but she told him no again as she had a boyfriend. Later that evening whilst my girlfriend was asleep I decided to check her phone. She had sent her sister screenshots of text messages from this other boy meaning she had exchanged numbers. This to me meant she had lied about 1 part of her story and put the rest up for debate. I immediately woke her up and confronted her. Her defence was she was drunk, and only saved those messages as he had said nice things about her. She said she loves me with all her heart. I demanded her to tell me the truth. I told her if she doesn't want to be with me anymore then she could stay at my house until she found alternative arrangements and id sleep on the sofa or alternatively she needed to stop lying to me, tell me the truth as I'm a massive believer of trust and communication. She swore blind she didn't kiss him or anything like that they just talked and held hands. She told me if we broke up shed be devastated and wouldn't know what to do with herself anymore. That's why she lied Thoughts?
My girlfriend of 1 year 3 months went out with sister clubbing and held hands flirted and exchanged numbers with another guy and tried to cover it up?
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