Whats this pain in my left calf after i run?

Im 23 and have always done athletics & long distance running since I was little, I still do it now to keep fit. But in the past few weeks iv had this problem where after i finish my run my left calf hurts for a couple of hours afterwards..sometimes much longer. its not completely unbearable but its like a dull constant pain and i dont think its muscular. As iv really tried stretching it out but nothing ever works. I feel like wen i stop running i can feel all the blood goin back into my legs wen i finish! not sure if this is normal cuz iv never actually had this problem before and its worryin me! not sure whether to go to the docs? anyone else had this problem or know what it cud be?