Coke doesn't have as much caffeine in it as coffee. You can have a cup or even two cups of coffee every day (which is the amount limit given by doctors). Don't worry about what you drank, that little can't hurt the baby. You could have one every week and it wouldn't hurt the baby......there's nothing wrong with having a little self-indulgence every once in a while. Just remember to be moderate.
The only thing that maybe you would want to be concerned about is that coke doesn't have any nutritional value and that you really need to have water instead. So do have a glass of water too. Coke also depletes your system of vitamins. So drinking it too much, like more than once a day, combined with the caffeine, high sugar, and empty calories, high sodium, probably isn't good for you. If you need to have one, you can also try drinking "diet right", which is a healthier option: no caffeine, sugar, or that wonderful bloating thing: salt.
Don't feel stupid either. It's normal to fret about things when you are pregnant. It just means that you want to do what is best for the baby