Hi!
Sorry it's taken so long for someone to get back to you.
When you say just below your breast bone, do you mean the area where the ribs come together? Is it tender? How long have you noticed it? Are you overweight?
Let me know the answers to these questions and I'll give you my best possible answer.
Dr. Eric
Most likely this is your xiphoid process at the bottom of your breast bone. The reason I asked if you were overweight is that the extra pressure in the abdomen tends to push out on this joint, and make it more prominent, leading some people to worry that they've developed a tumor. You can actually get arthritis in the joint which will be more tender.
Losing some weight will take pressure off the joint and allow it to relax. Most people know of this bone from their CPR class, because you are not supposed to press right on it to do chest compressions - it could snap off.
Here is a picture of the sternum with the xiphoid process:
http://www.probertencyclopaedia.com/j/Sternum.jpg
Here is a wikipedia entry on is:
http://en.wikipedia.org/wiki/Xiphoid_process
Hope this helps. Let me know if you have any further questions.
Doctor (MD)
9 years of experience as board certified internist, pediatrician. Specialize in Adolescent Medicine.