Milking an animal is an old image of drawing nourishment and reward from steady care. It can also be read as taking too much from a source, or being used.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.