Sentence structure "I'm [doing something] because [reason]"?
I think “因为。。。所以。。。” still applies in this case. Or you could say “我。。。是因为。。。”

For example: 


  我吃蛋糕是因为我饿了。= I am eating the cake because I am hungry.
  Wǒ chī dàn gāo shì yīn wèi wǒ è le