Which alarm went off?
If it was the low blood sugar alarm, it may have been due to a compression low. Libre (and CGMs) report a false low reading if you apply pressure which can happen at night if lie on your sensor.
The way around this is to apply future sensors somewhere you will not lie on such as slightly under your arm.
I am concerned that you are treating a high or low without double checking. Libres are renowned to be less accurate when high or low. The advice I was given was to trust the Libre at "normal" levels but always double check with a finger prick before correcting.