How a wrong bra causes pain
When the band is too loose, all weight pulls down on the straps, which dig into the trapezius muscles. This causes shoulder grooves, headaches, and neck stiffness — especially for D+ cups.
The 80/20 rule
A correctly fitting bra puts 80% of breast weight on the band and 20% on the straps. Most ill-fitting bras flip this ratio, putting the load on the shoulders.
Signs your bra is causing pain
Red grooves on the shoulders at the end of the day, neck or upper-back tension that eases on weekends, headaches that start mid-afternoon.