If it is intercostals there can be all sorts of things to set it off. It could be your bike fit - I am not experienced enough at biking to rule that off. But mine tend to get bad at my computer. They get stiff and then something like a coughing fit will set them off. I can imagine easily a scenario where being in one biking posture locks them up and then moving makes them cramp. Or having them be tight from another posture and then biking posture setting it off.