Calf pain when running?

my left leg is good to go, never gives me an issue. My right leg..the lowest part of my calf, hurts after running, and its tender to press on with my thumb...just wondering if anyone has any idea why this could be. Its only right lower part of the calf. I didnt know dehydration, but im well hydrated. I didnt know lactic acid buildup, almost feels like a cramp...but its not really a cramp...its just really sore now and i dont know why. i have been fitted for shoes, i have stretched proper before and after running, i am hydrated...just dont know what is up? Maybe an MRI would help me. I just dont know.