What did you eat after your fast? How much did you eat? Sometimes fruits with high Sugar content can do that as well as large quantity of your meal. Most Important part is to know what is the Sugar level after one hour, two hours and three hours after you ate. If it does not normalize quick enough then you have insulin resistance.

Only thing I can think of is that your body had gone in Ketosis and then you gave it a jolt by eating which raised your blood sugar.

Let's see how you do in couple of days.

Good luck.

Archus