Dana Franklin feels dizzy and overcome with nausea just before she travels from 1976 back to 1819 for the first time. These are the feelings that are directly described. She is also no doubt fearful and disoriented since this is the first time she has gone back into the past and has no idea what is happening. It does not take long after Dana's second trip back in time before she realizes what the triggers are for her time traveling. When her ancestor, Rufus Weylin, is in danger of dying in the past, she is summoned in order to save him. Rufus has to survive in order for Dana to be born. She is returned to her own time when her own life is threatened in the past.
When she is removed from her own present time she does not know that it is about to happen as she cannot see what is happening to Rufus in the past. This makes the trips to 1819 more frightening than her return trips. When Dana travels back to her own time from the past it is far less troubling to her, both because she is returning to her home and because her life is in danger just before she travels.