Glucose testing for Gestational Diabetes and carbs?

I'm a little unsure of the difference in how sugars and carbs break down. I believe that sugars break down quickly hence the "sugar low" a little while afterwards. Then carbs turn into sugars a little while later?

I failed my one hour test by 18 points. They tell you to eat a breakfast just not anything high in sugar. I had a bowl of Very low sugar cereal but it's actually higher in carbs than a few of the slightly more sugary ones.

It was about two hours after the cereal that they did the blood draw, is that about the same time that I'd be getting a sugar boost in my system due to the digesting carbs?

I passed a 3 hour test at 22 weeks but still needed to be retested at 28. I'm just surprised that I failed by so much and want to know if the carbs may have been the cause or if I should be worried about having GD?

Any advice on how sugars break down would be very appreciated. Thanks in advance.