I was seeing a girl we had one minor argument, she distanced herself. Started seeing my former best friend while stringing me along (neither of them told me about it), repeatedly lied that she's in the hospital, that she's traveling, etc. then she texts me how she's sleeping with my friend right now. Dumps me with another lie that she' moving and then posts a picture with him.

Proceeds to circle back, through her sister and friends for several months to meet up with me while dating him.

Tells my friends we were technically not in a relationship.

I am perplexed by the logic and thought process of this. Is this mental illness or simply emotional immaturity?