The other give away that this is not a hamstring injury are the sciatic nerve irritation symptoms (spans is leg muscle groups) I am experiencing. So this was just plain bad luck and speaks to the fact that I did everything else right in the warm up phase. Victim of over exuberant striding in a short sprint. Hopefully this will settle down soon.
And I did something you are not supposed to do, looked over my left shoulder at the 50m mark! No need as I had a 10m lead in the heat- that's likely what threw everything off.
Walking 25:00  2.0 km (12:30 / km)
Ambling around Old Colorado City (bought a 1950 German toy train set I saw lurking at the back of an antique store- boys and their toys!)
Swimming 10:00 
Not overdoing it, however swimming BS caused me no discomfort in the butt region. Aqua stretching afterwards.