Thanks for your query.
The pain that you have is likely to be due to a muscle sprain
or a muscle fibers tear due to an injury
or exertion. Lump could be due to muscle spasm. MRI scan is the investigation of choice to detect any such soft tissue
In the meanwhile, for symptomatic relief:
1. Give enough rest to your hand and arm muscle.
2. Take mild pain killers like acetaminophen
3. Stronger prescription NSAIDS with muscle relaxants may help if simple analgesics do not help.
4. Hot compresses to your arm will be useful pain relieving measure. You may apply a local analgesic gel on that area.
I do hope that you have found something helpful and I will be glad to answer any further query.