Ive never come across a false low in anyone except those with severe circulation issues (ie testing old blood) and as a Paramedic,...there aren't many cases of that And blood sugar testing isnt high up our priority list in those cases
She sounds as though she was symptomatic of low blood sugar last night-shivery, grumpy, tired, weak.
Best thing to do in those situations is as you did - quick sugar release (juice) and some carbs to build the stores back up - sandwich.
We often encourage people to have a sandwich/toast, a banana, some long release things.
Without another testing strip, I would be making sure she felt back to her normal self, and that she had had plenty of extra carbs to rebuild her levels up to normal for then, and to ensure they stayed up.
And just make sure going forwards you have one spare pot at all times for the testing strips! As soon as you open the next one, get a new pot for spares!
Obviously if there were any concerns in that she wasnt feeling better or not back to her normal self and you had no strips, you can always phone 111 out of hours, and in an emergency 999.
If this is highly unusual for her, in that she never has low blood sugar like that, and there was no clear cause for it then you need to get an on the day appointment with the GP surgery, preferably with the designated diabetic lead/nurse as it may be she needs further testing/investigations or medications