Best to let the vet have a look - and make sure they give a diagnosis and treatment, not just leave it and see. It could be when she grazed her udder, and an infection has set in. If it is mastitis, yes, there is always the danger they lose that side of the udder, as in it does not have milk in it, but, one of my British Alpine goats had mastitis in one side, and it was really bad. I was sticking a needle up the teat (not nice but had to be done) every day, and trying to milk her out. Not easy when they are in pain, and it was coming out like cottage cheese.
But, the good news was, Penny went on to make a good recovery, and the following year when she kidded, she did indeed have milk in that side of her udder. And even better, never had mastitis again.