Welcome to NeuroTalk:

Basically to address your question... impaired glucose tolerance begins before diabetes is diagnosed.
So things may have been brewing before you started the metformin.

Metformin also blocks B12 absorption in many people, so I'd get a B12 test ASAP, and it should be 400 or above. Don't accept "normal" from your doctor, because the ranges go down to 200 and that is too low for nerve health we know now (for over a decade). The lab ranges just have not changed, and doctors are robots when ranges are printed out by the labs. Low B12 levels cause nerve damage.

Methylcobalamin (the active form of B12) is the cofactor for melatonin synthesis from serotonin. If you are low, you cannot make melatonin the sleep hormone for yourself. Therefore methylcobalamin is the ideal form to supplement with if you need it.

You should also get a Vit D level done too. And come here with those results as well, because the D2 the doctor will give you has been found to not work well. You would need D3 OTC instead.

Those are the first things you should look at.

There are other supplements for diabetes, but get those done first and out of the way.