Were you lactose intolerant, or did you hold in going to the bathroom? If so, you can damage the bladder inner wall lining, and it will bleed, and mix into the urine. Keeping your intestines moving and empty is an issue here, for if you didn't, it can do this from all the additional pressure on the bladder. One of my sons had this and had to see a Urologist for it. It wasn't peeing straight blood, but the urine was definitely like a diluted cherry Kool Aid color.