I have a headcanon that Pinkie doesn't actually exist in this reality, but in fact in a parallel dimension that intersects very closely with ours. Since these parallel dimensions are so close, their timelines mostly line up, and she can exist in both simultaneously, but occasionally events will differ slightly between them, and her brain will have to either deal with two full inputs, or she will blink out of one momentarily. Obviously, if a normal pony were to experience this, they would completely lose their minds, but because Pinkie Pie was born like this, she doesn't think anything of it, and she's gotten used to the things that go on around her not always making sense, because she thinks that's just the way the world works.
Pinkie Pie is actually a genius; her brain is just working overtime to keep things in basic working order.