From Buddy 144 (retired orth. surgeon). Most likely muscular pain, probably cramping from mild electrolyte imbalance, given the scenario you've described. Two squarisn muscles in this area that help connect the scapula to the spine, called rhomboid muscles (from their shape). Sounds like you need more water and salt when you do this type of prolonged stressful exercise.
Sound like musculoskeletal pain. You'd have to be examined by a physician to be sure. I don't think that any imaging studies are necessary at this point. I am assuming that you don't smoke or have any known connective tissue disorder.
From your description it sounds like its muscular but its best answered by the Dr here.
FYI, you can often get knots or trigger points in this area. When you next feel it, have someone press hard on the area with their thumb, if it increases the pain, then its a trigger point/muscle knot. Heat or ice are good remedies, then appropriate stretching etc. If you are riding hard on a bike, the body position alone is probably aggravating things.
That said, if you are a woman, then it potentially can be a sign of chest pain felt atypically.
If you are worried at all with this type of pain, then seek out an urgent care or make an appt to see your PCP for an evaluation.
Fiona