Okay... random pissing of blood ain't a good thing. I've become far too accustomed to the subject in the last few years having had it happen to me several times, first from a Linkin Park mosh pit, once from a kidney stone, another time from the surgery to *remove* a kidney stone and 2 weeks ago when I had surgery that noone really wants to know about. If you've been hit or had a recent trauma such as an auto accident, or a hard slip and fall where you landed on your rear it can usually be chalked up to minor kidney bleeding. Color matters. If it's a consistent color, ie, "wine" then it's likely from the kidney or bladder. If the red is brighter or "unmixed" it's from an erosion or trauma to the urethra. I'd suggest straining your urine for a few days through a coffee filter if it happens again. Look for particles of black or dark orange, those would be kidney stones, and if you see them you take em into the urologist to have them analyzed. If you feel lower back pain when this happens, that's probably what it is, but either way you need a renal panel to confirm kidney function, uric acid output, etc. And anyone reading this - seriously, stop with the Coke. Ever pour Coke on a corroded battery terminal? Think about what that does to your stomach lining which has a considerably lower "metal and vulcanized rubber" and a higher "squishy flesh" composure than the battery terminal.
Best of luck to you,
Johnny "Frankencrotch" Swell