Anyone recognize this?

Usually I eat things that are not paleo. Not much but something all days. It could be a slice of sour dough bread, a small piece (40-50 grams) of boiled potatoe, a little youghurt for sallad dressing, or something else. I always use a little milk in my coffee but I only drink a jug of coffee for breakfast. 

When I try to go completely paleo it works well for a day or two then I get a pain in the kidney area. It's like i tired aching but I don't know if it is the kidneys or something else or just coincidence but it makes me worried.

Has anyone experienced this? Am I doing something wrong? Too little or too much of something?

Eva