Katie started feeling a rather warm sensation coming from her lower belly and crotch area. The feeling got more intense and Katie thought she might have an orgasm right there if it kept up.
It then stopped suddenly, and was replaced by a full feeling. Her shirt and pants then started getting tighter and tighter. Eventually they bulged out.
"Ugh this is so uncomfortable" Katie said as she pulled her pants down a little. A large pink vein covered sack then flopped out onto her lap, it was almost as wide as both her hips and ended with four small nubs.
As both girls watched the nubs grew longer and longer until they became teats.
Then a sound not unlike rushing water seemed to come from the udder. The sack began filling out and expanding, rounding out. As it did it became heavier, Katie had to open her legs wider to allow it to hang without discomfort.
When the growth finished the veins began throbbing, "Unnnngh milk me Tiff, milk me!"