Only one kidney left

5 years ago I had a cancerous kidney removed. Earlier this month I had my last follow up test. Blood tests and full body CT scans.All were good.The creatin level was good and I was declared cancer free.
Last week I saw a stomach specialist about persistent stomach pains. He asked me a lot of questions and examined me thoroughly. He prescribed Bentyl, and cautioned me about possible troubles I might have with urinating.He was so right. This morning I was having difficulty urinating, and then I had a terrific pain in my kidney area.(flank).it lasted maybe 5-10 seconds.Then it began to decrease in intensity and in a few minutes was gone.My question is...now what? Should I get my tests redone? I plan to stop taking the Bentyl,but I am concerned with the cause of that excrucuating pain. Any suggestions? I do have an enlarged prostate ,I am 71 years old. I have noticed that that is one condition that does not go very well with Bentyl.