Stress fractures are one of the most serious injuries you can get in triathlon or running. It usually require 6 to 8 weeks of rest and the use of a moon boat. It is possible to keep swimming for most stress fracture and it is even possible to keep riding on the flat or indoor depending where the injury is. It mainly comes from high volume, not enough rest and malnutrition. Here is a good article from The Injury Clinic about it. Train safe 🙂