Sound similar to a story I read long time ago and sadly can't find these story again.
It was about a girl who had some dispute with a stranger woman in a subway station about the strangers woman wide hips, as a result the stranger woman made a curse on that girl to even grow a "fat cow ass" and so it happens over several nights that her hips became wider and she was gaining weight on her lower body while up her hips stayed everything as slim as bevor. Until this she thought it only was some normal hormonal weight gaining, until a small bulge was visible on her lower belly.
This small bulge got four little domes and over some more days this bulge had become a full functional cow udder and for help she called her girlfriend cause she had become so huge that she had trouble to drain herself. So the curse of the stranger woman had tranformed her half into a cow.
In part 2 both girls had moved together away from the city to a small town where they lived together in a farmhouse and her friend had to milk her several times a day and take care about her huge body as her arms can't reach her enormous proportions. One day it happens that the udder-cream fell into the bucket with the milk and when she picked the cream out of the bucket she had her forearm covered with the milk and she couldn't resist to try the milk and it was so delicious.
The next days the girlfriend increases the consumption of her friends milk, first she got only one cup, but it increases till she was so drifting away that she drank the whole bucket.
This made her so tired that she misses the morning milking session and her girlfriend had grown her udder full of milk ready to burst.
Till this time she produces nearly twice the amount of milk as she hat till this day, but not only the udder girl had undergone some changes.
Also her girlfriend had grown, she a gotten bigger in her chest, but more important she got a small bulge in her pants.
But as she had no control about her milk consumption, her transformation continues till she had grown a huge pair of Zena Fulsom like boobs and a functional dick between her legs fitting the proportions of her cow girl friend.
And it happens what had to happen and both had sex and so part 2 ended with the information that the cowgirl got preg.
To bad I lost the files due to a hard disk crash and never was able to find them again nor saw a part 3.
The story had also a small number of illustrations involved, but most of the story was imagination
