Artificial Sweeteners Effect on Blood Sugar

Hey everyone...I’m “pre-diabetic” and just started tracking blood sugar using an inexpensive (ReliOn) glucose meter. I have been on metformin for nearly 5 days (500 mg ER/twice daily). After fasting for 18 hours my reading was 135 - the only thing I had was an ice coffee with Splenda and a vanilla flavor shot from Dunkin Donuts...supposedly no sugar. Shouldn’t my reading be lower on metformin...or could the coffee somehow have screwed the numbers?