Is there something wrong with my chihuahua?

I adopted a little 5 pound chi from the shelter in February. I love her to death. But she doesnt have any fur on her belly, or under her arms. She is 3 years old. Is this bad? Im pretty sure it has been like this since I adopted her, but I think it has gotten worse. There is litterally no hair there. Is it alocepia? Is she healthy, or does she need more vitamins? Please help! Am I freaking out for nothing? Thanks! -Sarah, 15