What Causes Extreme Pain At Back Of The Leg?

Hi, I have been having this stabbing pain in the back of my leg all day, It s not when I sit or lay. But when I try to stand up it starts, And walking is even worse. It has taken me over 10 minutes to walk to my kitchen and back to my bedroom. I know I am prone to getting leg injuries, But I haven t played sports in years! I just don t know what this pain could be, Like could it be a torn muscle. Or a tendon? It s scaring me!