Not sure, I just know that my dreams had always been so vivid and realistic, to the point sometimes thinking back I wasn't sure if it really happened or not. So perhaps you have/had a powerful imagination.
What was weird about my dream was I don't ever recall dreaming I had breasts. It was most definitely a pleasant experience. I think I'd welcome more dreams of that nature, although I may wake up disappointed a lot.
Hmm, an odd thought popped into my head, perhaps the vividness of your dream was infant recollection of nursing as a baby? That may somewhat explain having an idea of what the warmth and weight would feel like. As to the hard nipples, everyone's got nipples so that part is understandable as to knowing what that feels like. Well, just a thought.