Why is she being cold when I've done nothing to her?

I have sorta experienced this also its weird isnt it? its like all of a sudden things go bad well i think the main part of this is that she been talking to that guy u mention she may like him now and could be giving u signals that she doesnt want u to pursue her anymore i hope am wrong 4 ur sake but i could b the case dont think much of this because it will consume ur focus.