Should I trust my girlfriend with her friend (So-call)???

everyday she txts and talks to him especially at night. and one night I as pretending to sleep i hear them talkin to him but jus then she goes outside to talk to him. I try to tell her what was ur talk about ..and all she says is nothing much jus small talk (small talk over 2 hours) and she gets mad at me whn i look at her txts and while she's txting she is cautious of whos around and whn i try to look at what she's txtin she hides her phone.... yesterday was the worse i was looking in her phone and came upon pictures, i saw a picture of jus her (not topless) i kinda freaked...I told her wat was up and all she said was i was going to send that to u..weird (kinda thought she was sending those pix to her friend, So Called...i buy her gifts and cards and i know that she loves me.. is she's going through a phase wheres she's prob. bored of me?. I kinda believe that he is a friend but the things she does disturbs me. Should i worry? do something? etc. help. Thank you.