Would losing 12 pounds on an already petite woman and dramatically decreasing protein cause bun creatinine level to drop from 15 to 6 in 3 months?

Of course: BUN/creatinine ratio fluctuates wildly if you so much as eat a big burger. Don't pay much attention to your labs if they're near reference range. Your best weight is what looks and feels best to you. Glad you're staying trim. Best wishes.

Loss of muscle mass: In addition to losing fat, you are losing muscle mass and that is reflected in the drop in creatinine. You may wish to increase your protein intake and exercise. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Drink enough water daily so that your urine is mostly colorless. Practice safe sex.