This is going to sound a little weird, but chicken sometimes just does that. My name is XXXXX XXXXX read on and I will give you the other 2 probables on why this may have happened.
Depending on what area of the fridge was kept in it may have dehydrated a little bit and then further more when cooked in the microwave.
Most foods will
actually turn dark on your when you cook them, especially if they were already a little dehydrated. For example an egg based quiche will be light and fluffy, but if kept in the refrigerator uncovered will shrivel up and go darker yellow, almost brown when its left in the open fridge.
The other explanation is the blood vessels within the breast meat may have expanded when microwaved(like marshmallows and eggs and all types of other food "puff") and anything trapped in them may have ruptured and just spread. Its perfectly normal and would be OK to eat this, even though its a different color - again though, assuming the chicken isn't past its expiration date. It may have just been a bit dry. You sometimes see this around the edges.
Chicken turning dark sometimes happens randomly, its not a fault of the person cooking especially when its seasoned with the basics like you have done, cooked on a regular setting for a regular time as well as being the easy to cook boneless, skinless.
To prevent this dehydration though make sure you cover the chicken in the fridge in tupperware or wrap it in foil. When cooking do the same(though don't cook in the foil), make sure its covered in something microwave safe and if needed you can put a couple drops of water in to help create a moistening steam effect.
I could really go for some baked chicken right about now