What you describe sounds very much like a rib-muscle issue, especially since the blood tests were all negative for liver injury. It is odd this should have persisted for so long, but you may have gotten some inflammation of the connective tissue where the rib attaches to the sternum (costochondritis), which can become chronic if not treated. The usual treatment is simply non-steroidal anti-inflammatories like ibuprofin or naproxyn (Advil or Aleve), and sometimes stronger, prescription doses of the same medications.
If you've already tried over-the-counter meds such as mentioned above and they haven't worked, whether or not you bring this back to the doctor's attention would be guided by how much the soreness bothers you. It doesn't sound like anything serious and I think you can safely assume your liver and other internal organs are fine.
I hope this is helpful.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist