Could be insulin resistance. If your body becomes resistant to insulin your blood sugar doesn't clear out for a while, then your body basically supercompensates, puts out a ****load of insulin and your cells finally pull the sugar out of your blood but so rapidly it can cause a crash. I'd honestly stay away from the sugar if I were you, at least until a doc checks you out. If they test you for diabetes or IR you're going to have to fast for a while before the test anyway, so might as well get used to saying goodbye to the cinnamon toast crunch. The crash after the protein could just be a lack of low GI carbs leading to just plain old hypoglycemia later in the day, the good old afternoon slump which can be avoided by eating balanced meals throughout the day. If it is IR, the sugar you're eating is getting into your blood, staying there longer than it should, and then being pulled out at a rate faster than it should be, leaving you with low blood sugar which you'll generally start to feel in the early afternoon. Just another crash really, not from having your blood sugar cleared too rapidly but from having low blood sugar altogether due to a lack of intake.