What is the relationship btw elevated creatine kinase, sertraline, and working out?

May be cause/effect. Unlikely for sertraline to affect muscle directly, but exercise, especially weight lifting can clearly result in slight elevation of muscle enzymes, and not fully clear from your description extent of elevation or your approach to exercise. These are not uncommon issues, and typically are of little consequence. But if you are also losing strength and stamina, consider changing your meds.