Why my girlfriend pays more attention to her friend then me?

i been going out with her for 3 in a half years.she started this new school and meet this girl they been friends for about 6 months.there best friends she tells her her problems rather then me and just talks about her to me she dont go to school if her friend doesnt and when i tell her about her friend she gets mad and says shes gonna go do stuff but like some dum stuff i dont no if shes serious or not..what do i do i need help on what to do?