Three weeks in really not a problem for an adult Ball. It could be that you have been feeding her larger food items and she just needs to "catch up" or it could just be the season. Quite often they will stop eating during the winter or breeding season. Just keep offering her a meal every week to two weeks and she'll eat eventually. You should only be concerned if she starts to lose weight or exhibits signs of stress such as frequent shedding or symptoms of an URI.