You've very young for this to be happening, and I would be willing to bet money that you either are experiencing a side-effect from meds, some sort of food allergy that causes brain fog, or do you have anything like fibromyalgia that has cognitive issues that go with it.

Google [name of med], side-effect, cognitive problems or something like that and see what comes up.

Pay close attention to when this happens to see if maybe you've eaten something different that might be contributing to it. Lots of hidden food allergies are so subtle they don't make you sick per se, they just cause things like brain fog, bloating, a runny noise and the like that are chalked up to other things. You might eliminate wheat, rye, oats, barley and dairy products from your diet for a week and see if that helps.