My daughter was 23 months when my son was born, and one thing we did for her is we bought her a baby doll. She had fun taking care of her "baby" while I was taking care of my son; she even lifted her shirt up to "feed" her baby while I was breastfeeding my son. We got her the baby doll a few months before her brother was born, and I played with her a little bit (showing her how to hold the baby, put the baby down carefully, feed the baby with her bottle, etc.) and she had a baby stroller to push her baby around in. She loved her baby and took it with her everywhere we went. It's nice because it's a way to get her used to a "baby," it encourages imaginative play, and it's a new toy for her.
Also, it might be nice for your husband to take her out on a father-daughter outing. It'll be special attention directed towards her and encourage her to bond with him. And if you have any relatives nearby (like grandparents) you could ask them to take her out to the playground or something fun so that you can get some rest and she can get used to having fun with other people (not just Mommy all the time). After the baby is born, you could try to make some time every day (even if it's just ten or twenty minutes) where you are completely focused on her (while Daddy has the baby, of course). She probably just senses that something new is going to happen and not sure how to handle it; once you have the baby and she adjusts, it'll be better. I know some people say that you shouldn't have children so close in age because they'll fight, but it always depends on the children; mine are pretty close and play together all the time (and, of course they fight as well, but no gaps in age will prevent that). Good luck, and congratulations on the new baby! :-)