It sounds like you are having a very trying day. I wonder if you started a food diary? It could be something in the birthday cake causing the behaviour (or making it worse), maybe the sugar, or coloring from icing. I know birthday cake can be a trigger for my boys too. It's a hard one to avoid though, but at least they each only have one birthday a year.

I hope the birthday has gone well anyway today. Has the little man had a good time?