It's normal for one's only child to have many conflicting emotions regarding the upcoming birth and eventual arrival of her younger sibling. There are reasons for an older child to be excited about a new baby in the family but I can also think of many reasons for an only child to feel anger and resentment. Telling your child that you are having this baby so that she can be a "big sister" may be a strategy that backfires. There is a part of her that does not want to be a big sister. Why would she want to relinquish her unique status and share all your love and attention with another child? These feelings are only natural.
I'd focus, but not overemphasize, that you will love her just the same as you do now when the baby comes. Rather than focusing on what will change when the baby comes (new room, new bed), I'd reinforce what will remain the same -- she'll still snuggle with you and read stories before she goes to bed, she'll still attend preschool, she'll still go out for pancakes with you on the weekend, etc. Letting her know what will remain the same in her world will give her a sense of security.
Expect and tolerate some regression on your daughter's part when the baby arrives -- tantrums, sleep difficulties, toileting accidents, baby talk, etc. After the birth, kids your daughter's age may well vacillate between expressing positive and negative feelings about the baby. Don't brush off, or condemn, negative comments; ask her what's making her feel that way and empathize with her feelings. Even though it may be quite physically and mentally draining after the baby's birth, spending a little time alone with your daughter each day (like the "good old days") is probably the best antidote for her feeling unsettled and jealous of her new sibling.
I wish you and your family a healthy birth and an even richer family life. Thanks for writing.