Newly diagnosed with Type 2; how inaccurate are glucose monitors?

Just got diagnosed this week. 7.5% a1c, 123 fasting glucose, normal insulin - blood was drawn a month and a half ago. My doctor prescribed me 500mg of metformin daily (which I started yesterday) and to come back 3 months later for another blood test. I got a glucose monitor yesterday because I don't want to have to wait 3 months to see how I am doing. It is showing me at 80-90 before eating, using test strips from three different vials, one purchased from a different store.

From what I understand, metformin takes several days to kick in (and I would hardly expect it to make such a drastic difference on its own as well). So what's the deal? Are $20 glucose monitors really this inaccurate? Or does changing your diet affect your blood sugar in just a couple days? Dare I allow myself some hope that maybe the blood test from a month and a half ago was a fluke or got mixed up somehow? I keep telling myself that there is no way it could have been screwed up. What's a good way to find out if my monitor is bad? I can't find any businesses around me that sells control solution.