It's fine to use flexible syns occasionally. Don't worry at all.
The best thing to do is carry on with the plan as usual tomorrow and don't feel guilty or try to 'punish' yourself by skimping on tomorrow syns.
I think it's the guilt that gets you and if you restrict yourself too much you're more likely to 'fall off the wagon'. That's how it works for me anyway!
Hope you enjoyed your syns x