Stress Fracture?

[DLB]

Has anyone out there every had a stress fracture? How do these happen?

I have a stress fracture and severebone bruising on my knee cap, but I don't remember hurting it that bad. I didn't fall or bang it.... Could it be a calcium deficiancy? My Doctor was very baffled at how I have this stress fracture. I had surgery on that knee last year (for a bone bruise and a frayed ligament).