Up till yesterday I had only ever seen a low blood glucose once.
Hmm, did you check the libre results against a glucometer? The libre isn't 100% accurate, particularly at low levels, so if this is a one off I personally wouldn't pay much attention to it. If it seems to be happening again, check with your meter.
The other thing to be aware of is that non diabetics can have blood sugars below 4 quite often, just not very often.
In this interesting study (thanks
@Ronancastled for posting it on the forums)
Continuous Glucose Monitoring Profiles in Healthy Nondiabetic Participants: A Multicenter Prospective Study (nih.gov)
Typical non diabetics actually had bgs below 4 for 1% of the time, so for an
unmedicated nondiabetic a reading of 3.5 isn't that unheard of, though you might be feeling
hangry at that point.
(Note you need to divide the study's bgs by 18 to convert from US to UK units for blood sugar). The problem for diabetics on insulin (and some other drugs) is you can go much lower than that.
Honestly, it sounds to me that you have got your bg levels just where you want them. Congratulations.
Disclaimer : I am not a doctor and this is just my opinion, not medical advice.