Breastbone/cartilage pain?

As for background, I'm a 25-year-old Caucasian male. I've been an on-and-off cigarette smoker for about 7 years, never more than half a pack a day. I'm going through another quitting cycle now, so hopefully we can make it stick this time. I've smoked pot in the past as well, but don't usually make a long-term habit of it (if I'm holding, I'll smoke a bowl or two in the evenings, but I'm rarely holding). I work in an office environment, in front of a computer most of the day, and my chair doesn't encourage proper posture.

On to my question - for a few separate periods over the last six months or so (generally lasting for a week or two), I've had some mild, intermittent pain and soreness around my breastbone, and slightly to the left and right, right around where the cartilage connects the ribs to the breastbone. It definitely feels like it's coming from the front of my chest around the bones, if you get me, rather than from deep inside the body, and if I push on my breastbone it's definitely tender.  

Maybe an important note - it's also not connected to my breathing. I can take deep breaths with no pain at all, and I haven't been short of breath doing daily activities or anything like that. I don't have any unusual coughing, dizziness, sweating, weight loss, none of those kinds of symptoms.

Can anyone relate to the above? I've been thinking that it could be related to my slouching posture at work over the computer, constricting my ribs and breastbone ... but! as any smoker can tell you, there's always that lingering fear. Could this possibly be lung cancer? If anyone has any advice, it would be appreciated.

- D