This is a tough situation. Parents often have a hard time dealing with their children's developing individuality and relationships. When factors such as race and sex are involved it can make the situation much more stressful and complicated for everyone.
You need to talk to your mom. You may not want to, and it will probably be uncomfortable, but you need to work things out. You say that your mom is focusing on the racial aspect of the relationship but a good part of her anger may be based in the fact that you had sex at all. You need to choose a time when both you and your mom can sit and discuss what is going on. This discussion will be charged with emotion and you have to do your best to keep a level head. Listen to her side respectfully and then present your side in a calm manner. You may want to use a trusted adult or relative as a mediator to help support the both of you.
Interracial relationships are always difficult for all parties involved. Conflicting perspectives often muddy the true relationship. That is why it is important to talk to your mom. Expressing your intentions and feelings will help her gain a better perspective and hopefully come to accept your choices. It may be very difficult at first, but you do not want to lose the very valuable relationship you have with your mother.
Remember that this may be a slow process and one that will take time and understanding. Talking to a trusted friend or adult can help you deal with your emotions during this process.