“Musculoskeletal discomfort?”

I'm a 36 (almost 37) year old male, 6'2, 167 lbs, non-smoker, not overweight, casual drinker. In mid-February (it's June now) I started noticing what felt like a strained muscle in my upper right chest area just under/around the collarbone, but it mostly only occurred when taking deep breaths or sort of "activating" my chest muscles with certain positions or movements. I am fairly active athletically, so thought perhaps I'd strained something. However, it never went away... and it wasn't sensitive to the touch, really, like a sore muscle would be (if I press with some firmness near the area I can sense some mild discomfort though). I notice that in some positions I can almost assuredly cause some discomfort (such as the pushup position, just for example). Now in June it additionally sometimes causes some neck discomfort and every now and then some slight discomfort near my right shoulder blade, especially if I sleep on my right side (so I try to avoid doing that).

I was diagnosed with GERD in 2013 but haven't had significant flair-ups for some time (only if I really overdo it with certain food or drink). In any case, I've taken some PPI's for the last month or so...it hasn't made a difference. I have continued to train athletically - not overdoing it, but I guess that's all relative. My main activity is running and cycling, with a bit of weight training every now and then. The pain is not sharp. It is dull. I wondered if it might be persistent costochondritis. I can pop or crack my sternum dozens of times per day (which sometimes helps the discomfort just slightly), especially when straightening from a period of slouching. This has been going on for years, with varying frequency (but never goes away). But from what I understand costochondritis means that the area would be sensitive to the touch (which it isn't, or not much).

I am concerned that this is some kind of cancer - perhaps lung or - due to the GERD, esophagus cancer (or something else I don't know). Could a growing tumor be causing this? Could it be musculoskeletal? A doctor when I was a teen suggested I might have a touch of scoliosis...but it's never caused problems. I scheduled an appointment with a new family doc, but it's only in Sept - as there's a delay in seeing new patients.