A few different reasons could be causing the red marks. It could be that your bras are too tight and are digging into your skin.
Another possibility is that your bras could be too loose and as you move throughout the day it could be rubbing and causing irritation.
Another thing to check is whether you have changed laundry detergents. Because the bra is next to the skin the entire day, and because it gets pretty warm under the breasts, it could be causing an allergic reaction to your detergent.
It could also be the underwire itself. If you have a nickel allergy, it might be that through washing, the protective coating on that wire has worn away in places and is causing a reaction. If you have a sensitivity to nickel, and many women do, that may be the culprit.