It could be inflamation of the cartilage connecting the ribs to your sternum, a condition called "Costochondritis". It usually goes away in a few months or a year. It's not usually serious, from what I've read, but it's always good to mention it to your doctor to have a look at it.

Here's some info:
http://www.emedicinehealth.com/articles/5058-1.asp

http://www.healthubs.com/costochondritis/

I've had this for about three years at differing levels of pain, but never more than an minor annoyance onece or twice a day. I never saw a doctor and it never went away. If I had seen a doctor early on, and if he was sharp, it might have led to an earlier diagnosis of an associated and more serious autoimmune disease I've been recently diagnosed as having. If I started taking medication for that three years ago, it would have saved my liver some of the damage it suffered over the last few years. The autoimmune disease probably explains why it hasn't gone away, though I'm still not sure I understand the connection.

In your case, chances are it's nothing at all serious. Maybe you fell or had a virus or did too many dips, but if it doesn't go away, let your doctor poke at it a couple times just to be sure.