My urine creatinine was 20 when 4 months ago it was 73, my bun level was 7. what could be causing this?

This means nothing: Urine creatinine varies with your hydration. You excrete a certain amount daily depending mostly on your muscle mass. Drink twice as much water, and you cut your creatinine in half. It's worth measuring only as a standard against which to compare other elements measured in your urine. A "low BUN" means nothing. You're not a kidney patient. I'm sorry no one explained this to you.