Fat takes longer to break down in our bodies and doesn’t turn into glucose the same way carbohydrate does. Most items in a fast food restaurant are high in fat (burgers, fries, milkshakes, etc.). Even the foods that have a lot of carb still have a lot of fat, which slows down the effect of the carb on blood glucose.
So if we take our insulin for all the carb, but don’t take into consideration all the fat, we can end up low pretty soon after eating. The timing of the food and insulin hitting the blood stream doesn’t match up; the insulin hits at its usual time (Humalog, Novolog and Apidra start working in about 15 minutes and peak in about 90 minutes) but the food hits much later (while carbohyrate, on its own, starts working in about 20 minutes, the fat slows that way down and the timing will vary up to hours).
It takes practice and observation, but luckily there are ways to deal with this. Those of us who use injections (syringe & vial or insulin pens) can take a little bit of insulin up front and the rest after the meal. Or we can take all of it after the meal if we are low beforehand. Those of us who use insulin pumps can take advantage of the extended, square wave, or dual wave bolus (which delivers the meal bolus over time or some right away and the rest over time).
Next time you find yourself at a fast food place, a little extra planning could prevent an unwanted and unnecessary low blood glucose afterward.