I would also suspect that porridge is the culprit, have you had porridge a lot before and test afterwards. Oats do seem to be an odd one, some people can eat them, some can't (I'm one of those that spike on them) I have also seen it mentioned that it can depend on how they are prepared and quick porridge oats that are finely rolled are one of the worst. If you like porridge you might want to experiment, for me it is easier to do with out.
The other one is did you make it yourself from oats or was it a ready made variant, I just wonder if it was ready done, what else it might have contained.
Take it easy and don't panic, and try and do a bit of light exercise and the levels should come down, look on it as a lesson learnt not a failure.