What is causing a severe itch on the top of my left forearm under the skin, because there is no redness or any sign of dermal damage. i scratch intensely and there is no sign of relief. it's only in this one spot?

Itch/scratch syndrome. Sometimes these start as a bug bite or other event. When you scratch, you trigger local response that releases chemicals under the skin. This triggers a cycle of itching-leading to scratching to later itch & scratch. Sometimes a cool compress can slow/stop the itch feeling and help. Covering the area so you cannot trigger the chemical release also helps.