My bun/creatinine is 31.9 but i drink 2 liters of water every day. i have had a lot of kidney stones in the past. could this high number just be from kidney damage? thanks?

Need normal values for BUN/Creat for your lab. Would track back and plot as far as you can to see if trend. Discuss with your Dr as to what may be causing it. There are many causes and not stones unless they have damaged kidneys;

The ratio BUN/creatinine does not have much meaning about your kidney function. If the ratio is above 40, low volume or significant heart problem can cause that. If you eat a high protein diet or take steroids on the previous days before the blood test, the ratio is also high. E.g. BUN 30, creatinine 0.8, ratio 37.5, kidney function is likely normal, BUN can be high due to various reasons.

No. serum creatinine is the best common test for kidney function. If you were adequately treated and did not have prolonged obstruction from the stones their would be no or insignificant damage from stones. Keep up the water. You will be fine