As a Type 1 I would be very reluctant to set a high-level alarm at less than say 17 - my sugars often spike up after eating, and the libre will also (if scanned) later often show that the scanned reading was above the line on the graph. But validating the reading from the libre against blood test strips - particularly when low - is important.
Variation in readings through the day is common, reasons for going low include the results of past pysical (and mental!) activity. For real low readings it is necessary to treat them with sugars and perhaps a snack (but the libre seems to take ages to accept that a real low reading has passed becasue the bllod gets the sugars way before the interstitial fluid).
Low readings can also come from dehydration, and pressure on the sensor (e.g. when sleeping).
But if in doubt contact the diabetes team - might need to adjust the insulins and (for short acting) the ration of insulin to carbs.