If anyone should ever encounter the same gross problem that I had during Saturday's run, Emma has send me along the explanation for runners:
Peeing blood (Haematuria)
Blood can be present in the urine the first time a runner pees after a race. This again is a result of the mechanical action of running which can cause the two walls of the bladder to impact each other. Each minor impact causes more severe damage until there is sufficient trauma to cause bleeding. Healing of the bladder walls occurs within a couple of days. The chance of developing haematuria is greatest if you have an empty bladder at the start of the race. Although the sight of what appears to be a substantial amount of blood loss can be alarming, it is usually nothing serious and will clear up within 48 hours. Runners are advised to see their GP if it is the first time it has happened to them to eliminate the possibility of a urinary infection.
So no biggie! We keep running!