If the milk is lumpy, udder hard and hot etc - you will have mastitis there - so continue with the AB's for a while, and yes painkillers (as in anti-inflammatories) are necessary to give at least every other day. While she is filling her udder up, I would try and clear the infected milk, as in strip her out once a day. Once the infection has cleared, I would go to every other day and then reduce slowly. If she is to kid again at some point the mastitis may well return as there is a high chance that you will have residual infection in there.
I would completely withdraw any kind of concentrate, so only hay and water, nothing else. As she is not going to rear a kid she will not need any concentrate all summer, I would just feed her a good daily amount of branches - once she is properly dry and the mastitits has cleared up - along with hay/grass by grazing. She won't need any additional feed.
I would generally question the need for pygmy goat mix in a non-breeding herd, a low protein maintenance mix of shreds and some oats should be perfectly fine.