Why is my dog whining only at night?

I have a 15 year old mixed breed and all day she sleeps but she's become practically nocturnal. When my mom and I go to bed at night she sits in the hall outside of our doors and just whines and whimpers and pants literally all night. We aren't getting much sleep anymore and no matter where we put her in the house we can still hear her. Why is she whining so much and how can we make her stop?