I had one ewe which had a really big bag, very milky, she only had singles for years and her bag looked fine, one year she had twins and we noticed one of them not looking well, we took it off and found only one side was working. We let her rear the one lamb and then culled her the end of the year. I would say if it is only the one side and you can get her to dry up or get the mastitis out, then if the other side is milky and fine I would say let her rear the lamb and then cull at the end of the season. Otherwise the next breeding season she could have twins or trips and only rear the one. Plus she could be passing on weak bags, if she has one, to her offspring, so yes cull at the end of the season or now if she can't rear the lamb, depends how much weight she has on.