I have a vertical lump on my rib cage under my left breast.  Originally felt like a bruise and then a week later the lump showed up.  Very sensitive to the touch and about 3 inches in length.  No lumps in the breast area or armpit.  The lump is about 2 inches under the left breast.
Welcome to Chat Doctor. 1. The symptoms and description of the lesion seems like an infective etiology with collection within.2. But needs to evaluate the source, in this case after examining the lump, I would have asked for an ultrasound lump area.3. Ultrasound will help in assessing the nature of the lump, its extensions (bony involvement, chest wall involvement), its source - where it is starting from.4. A lateral x-ray to rule out bone pathology.5. After that if It's infective without involving the chest wall - a small incision and Chat Doctor.  If there is involvement of chest wall further CT might be needed. So get the investigation down and needful treatment. Good luck. Hope I have answered your query.Anything to ask do not hesitate. Thank you.