Is my girlfriend cheating on me?

ok my gf & i have been togethr for almost 2 yrs.lately (about a mnth)shes been kinda distant,now she says she wants to b able to go b with her friends whenever,thats all good but last nite she left the house at 7:30 to go "give her input" for an upcoming purse party.i tried to call her for something about 30min aftr she left,she didnt answr,she calls me back an hr later,mind u before she left & when i pickd her up from work she was super quite,when she called me back last nite she was happy as can b..she says she didnt have a signal when i called her,we have at&t for cryn out loud,NO RECEPTION IS CRAP,but then says she didnt have her phone on her,she didnt hear it.that phone is attachd to her hand,trust me..i dont know what to think,but i need advice,we got togethr when she was married & i was involved as well..u see why i have all these red flags..oh yea she said that her friends house is nrth of phx,so thats why she didnt have a signal,what did she do,walk in her friends house,give her input & leave..HELP ME