I Was Fine All Day Walking On My Legs. I

i was fine all day walking on my legs. i went outside and took a step down the three steps and all of a sudden i got what felt like a calf cramp. i tried to relax my leg and could not get the cramp out. i tried bringing my toes toward me and it was more painful. it has been 2 hours and my leg is thumping. the pain sometimes radiate up my leg but for the most part it stays in my calf. i am now sitting on the couch with my leg up with the heating pad wrapped around my calf. it is very painful. what should i do?