I'm usually very reluctant to go to the doctor, despite the common advice "Don't try to guess, see a doctor!" Pshh. You get in there, and he barely looks at you and doesn't seem to give a crap, and tells you to drink more water and get more sleep, and you get a bill. I think YOU are the guardian of your health and fitness above all. Doctors can help, but that doesn't mean you still don't have the right to refuse treatment when they want to cut off an arm to fix your achy testicle! I'm not trying to say everyone else here is wrong. Seeing a doctor can't hurt (although it will cost you), and maybe there IS something serious. But I think it's possible to know your body well enough to know that if you have a stuffy nose, you're most likely not dying of cancer.

But anyhoo, I'm inclined to believe you about thinking it is muscular. If it feels that localized and specific, I'm thinking it's probably the connective tissue between your sternum and pectoral muscles. My first week of commuting I felt pain in my knee ligaments. I didn't damage them, i just got a little too ambitious when accelerating and I strained them. Took a couple days off and I was fine. I bet your pecs are similar - you're straining them somehow, and they are achy.

Take a couple days off completely, maybe a week, and see how you feel.

I'm a cycling noob, so I have no advice as far as setup is concerned