What Causes Sharp Pain And Twinge In Calf?

I have been training for triathlons for 3 years with no injuries. the other day i went to run a 5k with a friend. as i was running to the start line from my apartment and about 100 yards from the start i felt what i thought was a cramp, no big deal, just stretch out a bit. My friend and i got in the starting shoot about 20 feet from the timing mat. the starting gun went off and i took about 5 steps and felt a sharp pain and a twinge in my right calf. I immediately had to stop running and made it to the side to sit down. It was difficult to walk home and 4 days later it is still painful although i can walk with little to no pain. I have a sprint triathlon in 10 days but am very nervouis to run, any suggestions?