Contacted by head of school regarding an issue - should I be worried?
I think you should be worried that you and your colleagues might have to go without pizza the rest of the semester if you've already (unknowingly) burned through the budget, and not much else.

You could also worry that in the future you'll have to go through a more formal approval process, which will probably loosen once whoever is involved in the approval gets tired of approving it every week and eventually delegates responsibility again.

To me it sounds like everything you've done is completely reasonable. It's somewhat normal for these sorts of things to fall through the cracks.

If there are any serious consequences that come about from this, then there is some deeper dysfunction in your department that is completely unrelated to anything you've done. There are plenty of things for a PhD student to be anxious about, I'd suggest trying your best to not make this one of them. Just show up to the meeting on time and be apologetic if necessary, and also be understanding.