I have a pain in my left forearm. It almost feels like I over used the muscle, but I haven't that I know of.?

Let's see? Often we strain or sprain parts of the body without realizing it. A movement is made, a slip or a stumble or lifting or what have you. You do not feel the effects until later when the sore muscle, the strained tissues or tendons become inflamed and the inflammatory process sets in. Usually ice and Ibuprofen will relieve minor to moderate problems. Sometimes heat. If lingers, b seen.
Need to see a doctor. It could be your heart because heart attack sometimes has radiating pain to left arm. You should get a complete check up with doctor.