Hello and welcome to the forum. Sorry you have this pain. You should see a doctor as soon as possible and if it becomes chronic pain and swollen, then the emergency room may not be a bad idea. It very well could also be a sprain which involves ligaments in the area. This can also have swelling, bruising and pain with it. Pain level is not always the best way to tell if it is fractured or sprained. You most likely will need an x ray. Please see a doctor to determine what is going on.
A repetitive strain injury (RSI) is a gradual buildup of damage to muscles, tendons, and nerves caused by repetitive motion and overuse. You had mentioned you had tried not using that arm for anything heavy for a couple of months. That is a good first step. You should continue to avoid heavy lifting with that arm, or alternate which arm you use, or take more frequent breaks. Speak to your employer or occupational health representative to see if you can modify your tasks or transfer to another department which does not involve heavy lifting. If pain and loss of function are limiting your activities of daily living, you should seek medical help. Your doctor may advise taking pain medications such as NSAIDs, applying heat or cold packs, and/or wearing elastic supports or splits. You may also benefit from rehabilitation programs consisting of exercise programs to stretch and strengthen the area as well as conditioning exercises to help prevent further injury.