There may be many reasons you behave differently with your family as compared to the outside world. Without knowing the specifics, it's difficult to advise you. But, I would start by thinking about what situations trigger you to act like a "bitch" with your family.
By looking for patterns in your behavior, you can better understand what about your family triggers your anger. Once this is clear, you may want to consider speaking with your family members individually about your concerns at a time when you're calm and collected. Chances are if you're coolheaded, you'll get a positive response from them. Then you and your family can begin to work on changing the relationship for the better. It usually takes two to tango and it's likely that they're fueling the fire in some way that they may not be aware of.
It may be, though, that after some reflection, you can find no rhyme or reason for your irritability with them. Sometimes when people feel secure in an unconditionally loving relationship, they feel that they can get away with bad behavior. No matter what you do or say, they have to love you. Unfortunately, frustrations that build up during the day may then be taken out on them. With acquaintances and friends, though, we're often on our best behavior, aware that they could end the relationship.
The best way to stop this cycle requires you regularly taking time to tune into your moods. Are you feeling angry, irritated or sad? If so, why? What happened during the day? If you do this prior to your interactions with your family, you'll gradually develop the awareness that your frustrations don't involve them.
As with anything, it'll take time to change your behavior. But, keep in mind, if you wouldn't behave this way with a stranger, why would you with someone you love? If your behavior continues, you may want to seek individual or family counseling to address the underlying issues.