Dairymen use a strip cup as an early warning of something amiss. It's just a little pad of rubber fitted into the top of a beaker type vessel, with a couple of spaces at the edge to allow milk to drain off the pad. A few 'strips' of milk onto the pad, prior to milking allows detection of any abnormal cells, which will be left on the pad when the milk slips through. Used daily, these cups can help detect mastitis early.
Sometimes, with cows with large udders, there can be 'injury' from the action of her legs against her udder, so it may be caused by slight trauma to her vessels within.
Certainly in mares, whilst they are in their foaling heat, there can be a change in milk, causing scour in the foal, so there may be similarities with cows.