What causes red blood cells to be found in a urine sample?

I have mild kidney disease, and have to get ‘pee tests’ every so often. My latest urinalysis results came back with the news that I have red blood cells in the urine. Apparently, that’s not supposed to happen. I’ve only ever had this result one other time, years ago when my kidneys were extremely stressed (I was pregnant. And NO, I’m not pregnant, now!) What mechanism of my body is producing this result? Is it always related to a kidney problem? Are there other causes, and if so, what are they?