so, my doctor says that I have Gestational Diabetes. Yesterday, I did not change my diet. I ate what…

so, my doctor says that I have Gestational Diabetes. Yesterday, I did not change my diet. I ate what I wanted. just to see what my blood sugar levels are. But my blood sugar levels were under what the said they needed to be. (I got 99 after eating lunch. my doctor gave me that it has to be less than the 120.) I just wanted to know like I guess if I do have it. or if it was somehow a mistake.