Have you researched night terrors? If I understand correctly, night terrors can result from stress and/or overtiredness and usually appear between 3-5 years of age. I'm wondering if the change in your relationships could be the cause of stress and discomfort for your child. Try googling "night terrors and toddlers" or "night terrors and children" to see if any of the descriptions remind you of your child.
A friend's son also suffered from night terrors at about the same age as your daughter. At the time, there was great tension in the marriage and I believe this was the cause of this child's stress. Today, he is fine. Hope this helps....
Does she have sleep apnea? One of my children did. She had narrow and inflamed nasal passageways.