Why did my girlfriend cheat on me when we had a break for 2-3 weeks?

Do you believe that it is possible to fall in love with someone in such a short period of time? I think she didn't love me... She says the opposite but I don't believe her :( Why did my girlfriend cheat on me in such a short period? What do you think?

Why did my girlfriend cheat on me when we had a break for 2-3 weeks?
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