It could be a lot of things. If it has to do with your heart, it is important to note that heart problems cannot always be detected under ordinary circumstances. Many well-trained athletes have found themselves in the hospital thinking otherwise.
If it always happens during a 400, then that kind of stress would have to be replicated during a test to rule out the heart. I need to ask, do you warm up before the 400, or is the 400 your warm-up? What percentage of effort is your 400 during these episodes? When I was coached, we usually did one easy warm-up lap clockwise on the outer track before doing our 400 timed repeats. If you are doing them cold-turkey, that could be part of the problem.
Meanwhile, breathing muscles can be stressed by non-breathing activities, causing similar effects under sudden demand like a fast lap without warm-up. Reaching overhead to access a shelf is one such activity. The breathing muscles for inhalation act to elevate the ribs to expand the chest, which is how straining while reaching can come back to bite you.
Another issue to consider is the gradual degeneration of intervertebral disk tissue that occurs later in life, which can lead to nerve impingement during impact activities. See if you can replicate these symptoms with another non-impact exercise at the same heart rate, to use as a diagnostic aid.
Back to the pain, in most cases a heart related pain will radiate upward and out ward into the limbs rather than down the back, but that’s no guarantee it’s not your heart. If you haven’t had a treadmill stress test, this would be a good time to schedule one. Not sure if it will be covered by insurance under these circumstances, but staying competitive into your senior years is worth the expenditure, just for the peace of mind alone.
Best of luck getting to the bottom of this problem, and I hope you enjoy many more years of training and achievement!