I'm always paranoid I have some sort of disease... so I want to make sure this isn't serious...

I only have one symptom... it's not really a pain but a discomfort right under my left breast. Sometimes I worry it's my heart, but my rib is also kind of sore there.

It comes and goes and is really mild; it feels dull and achy

Any ideas of what it could be?

Have you discussed this symptom with a family doctor? If not, then explain how long you have been feeling this and the location and get the medical opinion.