Not weird at all if your oats are ‘quick’ ones (and even if they’re not). In order to make them cook more quickly, e.g., 1-minute oats, 5-minute oats, steel cut vs rolled, etc., they undergo processing you won’t find in oats that take longer to cook, i.e., overnight oats! Not to mention, if your oats are flavored à la Quaker’s instant oats, they’re packed with added sugar. Yummy, but not advised, at least not by me.
The more processed the oats, the more quickly they’ll metabolize and raise your blood sugar. A bump in blood sugar is met with a proportionate rise in insulin release (assuming you’re not afflicted with some form of metabolic disorder) since its job is to get the fuel packed away and restore your blood sugar to safer, usual levels. Insulin release, in turn, increases serotonin levels since they’re packaged together in the pancreas’ beta cells, and I’m betting you can guess where I’m going with this now.
Serotonin is a mild sedative (and there’s something to do with tryptophan as a precursor, but I my understanding of the matter needs some refreshing) and your intake timing can make you nappy when you don’t want to be.
Anyway, pack a little protein with your oats and it will reduce the amount of serotonin release, even more so if you precede your oats consumption with the protein.