One thing to also think about is the type of food you're eating vs the insulin. The nachos were probably pretty high in fat and protein, which extend when the food spikes in your system. So even if you took the correct amount of insulin, it likely hit you before the nachos did.
With meals that are high in fat or protein, sometimes I'll take the shot immediately before eating or even a bit after starting the meal. Otherwise I'll go low, correct with smarties, and then end up high from the food + sugar later. This doesn't sound like it was your issue in this instance - but as you're new to insulin, thought it was worth mentioning.