Sorry to hear about your problem. Although tamoxifen
does lead to side effects like a clots in blood vessels and bone pain
, your symptoms are more suggestive of a pulled muscle. It does not seem to be a cardiac problem because you are not getting these symptoms while swimming.
You should try simple analgesics for a few days, but if the pain persists, you should visit your surgeon and have these symptoms evaluated.
Hoping for the best.