What do I do if switching the clocks changed the Ona, do I keep the time or the Ona? My last period came at 5:30 pm which then was the day, but now it’s the night.
You don’t keep the time if it moves into a new Ona. If it’s a day Ona, the Bedika must be done before Shkiah. If it’s a night Ona, before sunrise.
The time mentioned in this context is not on the clock, using standard time, but rather Halachic time, i.e. which part of the day it is and how close or far from the end of the Ona it is. Since the Bedika should be done at the same time, or after the time, of the onset of the Veses, it is best to do the Bedika close to the end of the Ona.