You should measure your temperature once a day, first thing in the morning when you wake up, before you snooze, lie awake in bed for a while, or get out of bed, to ensure you’re measuring your basal body temperature.
This is because the body temperature changes depending on how long you have been awake, if you have been moving, sitting still, and so on.
If you measure several times in a row, since basal thermometers can measure small changes in your temperature, your body temperature will naturally change, so the first temperature reading will be different than the next reading. If you have measured several times in a row, you should always log the first temperature in the Natural Cycles app.