It is common and well documented by Cahill in his landmark studies on starvation that at the start of carbohydrate restriction blood sugar often goes up in the first few days before it starts to drop.
I’ve seen this myself with long fasts (greater than 4 days). The first few days result in your body making a lot of glucose in spite of low carbs in your diet.
It takes several days for your ketone production to kick in. That’s why the body dumps glucose.
I’ve also noticed an association between weight loss and blood sugar. My blood sugar is often up on the day before I drop in weight. In reverse, my blood sugar is lower when my weight goes up.