why is my friend so jealous of me?

in no way shape or form do i brag about what i have, i mean i dont even have much, i live in an apartment with only two bedrooms with my dad, my mom died when i was 4, she lives in a huge house but she only has her mom, her dad died this year,yet she has the guts to tell me i dont understand what having a dead parent is like, but thats not the point here,so i love music, its my life, because of the money issues we have though i only get to go to one concert a year, im nice enough to invite her but instead she gets mad and says she wants nothing to do with it,then when i get back she looks at all the stuff i got there and gets even madder at me. then when it comes to my boyfriend, one minute shell tell me she can see us together forever and then the next she bites my head off for even saying his name, and then she will go and tell me how lucky i am to have him. then she bites my head off again for simply saying "i know" (ive been with him for almost 9 months.) and i cant bring her to my house because everytime i do she steals stuff from me, my other happiness would be my cat athena and the only time i brought her to see athena she tried to let her run outside and laughed about it, is she jealouse of me? or is she just a horrible friend?