Hello Andrew. This is Dr. Love. The other expert has opted out.
The most common cause of chest pain in a young adult is from inflammation of the chest, particularly inflammation along the joints where the ribs connect to the breast bone, called costochondritis. It is also common for the pain from costochondritis to be worse when taking a deep breath, because the deep breath is causing these joints to move a greater amount. The inflammation of one of these joints can also cause some swelling, and can feel like a bump, such as you describe.
It is also possible to get inflammation inside the chest that can cause chest pain that is worse when taking a deep breath, such as inflammation of the lining around the lung, but this is less common and would not cause a bump that can be felt.
It is a good sign that the pain is improving spontaneously, and it is not unusual that a bump from inflammation may take several weeks after the pain stops for the bump to resolve. There are a couple of home interventions that can be done to help. Since the pain has improved spontaneously, it is not necessary to do something for the bump, but if you would like to speed recovery, there are a couple interventions that can help. It can help to apply moist heat to the area of the bump 2-3 times per day for 15-20 minutes. It also can hep to take an over the counter anti-inflammatory medicine, such as ibuprofen or naproxen. Anti-inflammatory medicines can cause stomach irritation, and in someone with recent indigestion, it is reasonable to be cautious with the use of anti-inflammatory medicines. So, it would be reasonable to try the moist heat first, and you can consider an anti-inflammatory medicine based on the clinical progress with using moist heat.
If I can provide any additional information, please let me know.