I cant stop crying honestly ...?

my bfs phone bill came in he told me open it see how much it was i happned to look tru as it was itemised and saw he had txt a number a lot i rang the phone a girl answered asked him if he knew the number he said no i txt the number and she said she is his girlfriend my bf wont answer as he is in work im torn to pieces guys honestly wat can i do or would this girl be lying please be honest guys i cant stop crying here