Hurts again!

Someone told me that he saw my bf was msg with other girl behind me..
i feel so sad..
i can't believe that he do like that to me..
i love him so much..
i care about him so much..
how could him..
i can't stop think about this..
should i believe him or my friend??
should i?
but why he has to do like that..??
why can somebody tell me why..??