It means she is picking up your energy and realizing that you desire to feel deeply connected to someone or something and nurture and care for it. It refers more to an instinct, something you want in your life (to love and nurture and be loved in return) than a physical state of pregnancy.
Well, my mother-in-law dreamed that I was pregnant, then my cousin come up to me and said she dreamed that I was pregnant and had a boy, and guess what? A week later rolled around and I was pregnant...haha
She could either be wishing or worried for you. But dreaming pregnancy can also mean that she feels that you are holding some anger towards her or resentment.
It means nothing. Don't worry about it. She might have been thinking about pregnancy and then saw you before she went to bed. It's common to have dreams about people because of seeing them, hearing them,or thinking of them. If you are concerned then carry a condom around with you just in case.