Hi again.
4.5 isn't too low, in fact it is good, as is the 6.2 after lunch. I have just taken mine and it is 4.7 (about to eat evening meal).
Tiredness and thirst is normally associated with high readings I believe. If you have been reducing your carb intake this could be the reason your levels have dropped a bit. try again this evening, test before and at 2 hours after and see what happens, then come back and tell us. Try not to stress about it as this will almost certainly increase your levels.
I hope you feel better.