Why do girls try to make their bf jealous?

Ok i am pretty bad sometimes, i've been in one serious relationship and she cheated on me a year into it. My current girlfriend hasn't done anything too bad, i caught her in a lie well sorda lie she told me she was going to hang out with her grandma, and text me later saying she was hanging out with her guy friend. Can girls have guy friends who are nothing more than friends, i have never had a girl just be a friend, it hasn't worked for me something happens someone hits on someone always i think. She constantly ures me nothing is up but i get really jealous, and she will say things sometimes waiting for me to react, then say she is kidding and just wants to see me turn red or something. So damn irritating, like today somehow we talked about that night she was hanging out with dude, and a song came on the radio and she said i dance with a guy to this song, i said why would i want to know that and she said why do i care and i didn't say a word i just tried not to be mad, but im thinking she is immature and untrustworthy.