Pregnancy dreams are very common and often symbolic, especially given your circumstances and beliefs. Here's a link to the definition:
pregnant
The second type of pregnancy dream is probably the relevant meaning.
As for what the baby represents in your waking life, hopefully that will be obvious. If not, your dream is full of clues. The fact that your ex is in the dream suggests that it might relate to your romantic life... perhaps a new relationship(?) In your dream, the baby is a "newborn," so the relationship could be very young and new. The fact that your mother is in the dream and doesn't believe it, provides another clue. The fact that the baby is male, could be an allusion to the new male in your life(?)
Feelings in dreams are never disguised so the fact that you are excited about having the baby provides another clue as to its symbolic identity. Where do you feel just as excited in your waking life? The fact that you also feel ashamed to be living at home in the dream, may relate to your concern about how that might appear to your "new baby."
I hope that this leads you to a better understanding of your dream.
Good luck with your "new baby." May "he" grow big, healthy and strong!