Thank you for asking your query.
This particular lump, as per your description seems to be a infective swelling. Something like a infected cyst or furuncle. This is unless of course you have not hit/bumped your arm somewhere.
This should settle by itself in less than a week. You may apply some warmth (take care not to burn the skin) over the swelling twice a day. This can alleviate the pain and inflammation. Tylenol or any OTC pain medication can be used if there is severe pain. Make sure not put pressure over the swelling. You may also want to see whether there are similar swellings in any other part of your body.
You will need to visit a Doctor if the swelling is persisting and there is no relief within 2-3 or if there is excoriation of the skin and/or discharge.
Hope this helps