Have you worked any backstroke into your swim routine? If not, consider swimming several hundred yards of backstroke at the beginning, middle, and end of your workout. As a distance freestyler, I used to have localized clavical/pec pain like you described, and my coach ‘fixed’ this by simply adding more backstroke to the practice. I have a hunch the fix worked because backstroke caused complimentary muscle groups to be stretched and strengthened. Don’t take my word for it though, bounce this idea off your coach/trainer, and definitely take it easy if you are in a lot of pain.