Answer:
You're confusing things just a little like so many people do. So you're not alone. I.Q. is only a component of intelligence. Memory, Experience, Empathy(and socialization), Thoughtfulness are the others.
First you must understand what I.Q is. I.Q. or Intelligence Quotient was a test conceived in the early 1900's and became popular in the education systems of the 1950s. It was a way of categorizing the smart students from the normal population. Then the smart ones would be streamed into a higher educational path.
While other tests for intelligence been developed and rolled into I.Q tests the base of the test remains the same. Primarily it attempts ( and I emphasize attempts) to quantify problem solving ability. Thats it.
Too many people confuse intelligence with speed. It matters not if a person arrives at a solution twenty seconds later. It is the same solution. I.Q. originally tested the speed of problem solving ability.
The problem is that the I.Q. tests was specifically designed for young people of North American Caucasian culture who have an affluent, nutrient rich background. If you were poor. If your parents were Black, or First Nations, or Asian astoundingly these youth magically appeared to be stupider and nothing could be further from the truth. So in the early years, frankly speaking I.Q tests are racist. And that fact makes the I.Q test(s) useless as a measure of intelligence.
While the great psychoanalysts have tried to address the racial incongruity of such tests by adapting the tests the problem is that still different cultures and languages will effect the norm of the test.
Another problem was the test itself. It was originally designed to measure problem solving ability only. And the speed and quality of that person's problem solving ability is in theory measured to a standard of what.
Here was the original equation. IQ=100*(Mental Age/Chronological Age). This is fine as kids and youth because as originally conceived it was only meant to measure intelligence up to the age of maybe 20 or 24.
Groups like MENSA use the tests to brag it up. They evolved a standard of I.Q - 140 as the qualifying number on their tests. The problem with this is that they seem to have that number all the way through adulthood. So how does this work?
When a MENSA person gets to 85 years of age that person is equivalent to a person of 128? How do they get a proper demographic on that one? Did they measure the intelligence of a person of 128 years old?
So in adulthood all these alleged I.Q tests are basically clutter and utterly useless. I.Q test is only meant to measure people. These hideous things categorize people and peg them into a course of education or employment or social status.
And the great paradox comes forth. Anyone who brags about their I.Q. proves that they aren't smart enough to figure out that they are jerks.
Okay. Now like any measuring method like a blood pressure test can be affected by a myriad of factors. Yes I.Q tests can be effected by age. Yes the measure can be effected by any trauma or disease. Yes, I.Q. tests can be effected by the time of day.
Why? Well the brain is a part of the body. It is a use it or lose it like the muscle, or the bone. The brain can be dulled by a lack of stimulation or impaired by drug abuse. A lot of allegedly high I.Q. people became raging boozers and wound up just as stupid as me.
When my "educators" gave me an I.Q test in the morning. I was dead meat. No word of a lie. I would lose eight points off. Off what?
If the I.Q. test was given in the afternoon however. I'd gun the test and blitz the numbers. Like I had an ingrained contempt for these tests because I understood that they were trying to peg me into a program. Screw 'em. I'd do my best. And in the early years they tended to use the same tests and I had a better than average Memory.
Not only that a bunch of kids and one family in particular scored into a treasure trove of the tests left on a shelf by a lazy teacher and they actually memorized them. They were near geniuses. The biggest contention by the test manufacturers was that you couldn't study or memorize them. That is just a lot of hooey. Yes you can.
So big was the score of these "I.Q" teachers treated their essay answers like gold. They could give any meaningless answer and they would get the mark. Not only is I.Q tests bad for the students it is bad for those educators who believe in these absurd things.
Returning to the feeling of getting dumber. That is normal. When one is a kid under 24 years old, you are the smartest thing on the planet. As you get older, as you use Memory to accumulate that Experience one sort of goes. Hey. Holy Crap! Did I just do that?