I fell and landed on my forearm. at first i didn't notice anything but now there's a pretty noticeable lump on my left arm. it's sensitive to the touch but otherwise doesn't impact my mobility at all. what can it be?

These are often a bone scuff that triggers a local calcium deposit or a blood collection that can accumulate a calcium deposit. Either can take months to disappear as the reaction fades. If it doesn't bother you ignore it. If it persists or grows check it out with your doc.

A hematoma will typically form, an encased collection of blood that may clot and persist indefinitely. If causing symptoms or growing seeing your doc would be best as diagnostic imaging will likely be needed.