Medical advise please?

I have a back injury from a car accident a couple of years ago. Pain is shooting through my knees down to my ankles. I do drink a lot of mountain dew, wild cherry pepsi and pepsi like 5 cans a day. I don't drink milk or water much, and I eat once a day because I am so broke I can not afford much food. Most of my money goes the mortage. Why is my knees and ankles hurting so bad?