One of my goats did this a couple of times, took the bit to a dairy farmer and he said sometimes bits slough of the inside of udder/teat.
He showed me a horrible looking corkscrew thing they put up the teat to help clear it if it becomes a problem.
he said not to worry. looked like a tiny bit of cottage chees and felt like hard rubber.
Theres been nothing else since then.
Could she have knocked her udder or teat somewhere? I've realised mine take a short cut round a gap in a stone wall and the base stone is still there, maybe just high enough to catch.