I have the worst right flank muscle spasms. What causes them?

I have severe pain if I lay on my right side.  I also have severe spasms in the right flank area so significant that it causes me to urinate at the same time. Its a horrid pain and only happens after I have gone to bed and have to get up to use the bathroom. As soon as I stand up and take a couple steps,, bam! i get the spasm, it doubles me over and I end up urinating. Please tell me what this is.. ??? its so painful. I am afraid to go to bed at night now.