Hello
I have gone through your question and understood your concern.
Low energy can be a symptom of hypoglicemia and hypoglicemia can be a side effect of Humolog or Lantus.
At the other side unusual
tiredness, or weakness can be a symptom of
lactic acidosis,a side effect of
metformin.
Other symptoms of lactic acidosis include abdominal or
stomach discomfort, decreased appetite; diarrhea; fast or shallow breathing; a general feeling of discomfort;
severe muscle pain or cramping.If you do not have any of this symptoms (apart low energy),Lactic acidosis can be excluded.
I suggest to check your blood glucose several times every day(for some days)fasting, 2hours after meals or random to see of you have hypoglicemia.
In this case your insulines and your diabetes is the culpit of your low energy.
If you do not have hypoglicemia and lactic acidosis,i suggest to consult your doctor for further investigations.(If the weakness persist)
Hope i have answered to your questions.If you have other queries,feel free to ask .(you can ask me at HealthCareMagic at my private link: bit.do/hcm-Dr-Xhardo).I will be happy to help.
Take care.