Why best friend offended? Before you figure out how to reconcile, remember that could be in your words such that the person with whom you've been "thick as thieves", has ceased to communicate with you. After all, best friends do not pay attention to the little digs, they offend only the really serious words.
Often the best friends resent the distrust. You hid something, and the friend just had to know. Her having the thought that your friendship is not so strong as it seemed. She resents the fact that he was with you more honest than you. If this is your situation - explain to a friend why you didn't share with her. Tell us what you end up undecided about their attitude to the problem and wanted to tell my friend once, decide it for yourself. Or explain that you were ashamed of your behavior. Although who does not like the best friend you can voice your most stupid things.
Often best friends resent the fact that you began to pay less attention to them. Mostly, this happens when you have an ongoing relationship with a man, and the girlfriend is left alone. Try to understand her. Before you were always together, and now you have a pair, and she is lonely evenings and weekends. Do not let your friend be bored. Try to help her to arrange a personal life, married the guy friends. Arrange joint gatherings. Well, do not deny your best friend in communication. One night a week you can devote to your meetings will not be outsiders. You will be able, as before, to talk about everything and remember the fun days when you were together more often than now.
Girlfriend could be offended at anything, but if you want to keep communicating, be sure to try to talk. Frank conversation will put everything in its place. Express everything in your heart, share your feelings. Let a friend discover what's bothering her. You can even cry together - it unites. And always believe if your friendship is real, then resentment will soon pass, and you'll be together again.