If she has been "pensioned" off from a milking herd, but is still qute young, the most likely reason is mastitis. If it is sub-clinical it would mainly show in her milk yield, so a commercial herd would be more likely just to retire her....
If you are after a goat that gives enough milk for you and her kid(s) you really need to check out the parents and their milking figures, a kid needs about 2ltrs per day for at least two months, so if you have twins to feed you would need a goat that is capable of giving around 5 ltrs per day...
If you want milk for the house it is also best to ake the kids away and bottle them and strip the nanny out twice a day as Beth said above. That's what we do, I take the kids off anytime after day 4 (disbudding) and before 1 week old, but check the udder even before that and ease out if t feels tight.