What Causes Severe Pain In The Left Arm And Thigh?

My left thigh feels really weak and has a tugging sensation almost. And if i don't prepare myself before i walk i almost "fall" in a giant limp almost. Sometimes my arms feels like i want them to crack but i can't get it too, and i think that would be a bad thing to do anyways. I don't understand why it sometimes hurt and sometimes doesn't. Half the times when this happens, another part of my body begins hurting in the same way as the previous area stops with the pain.