What's wrong with my Hammy?

Olympic Tri over the weekend. On mile 18 of the bike my left Hamstring went... very top, right under my butt. Almost DNF'ed right then and there. Limped it home on one leg. Couldn't walk off bike it hurt so much. Forced myself to run and after mile 2 it actually felt OK. I can certainly feel it today. Something is not right. How do I fix it?