Unfortunately, your symptoms could be caused by many different things. Have you had a endoscopy and/or colonoscopy yet? If not, you probably should, but do a thorough background check on the doctors you see. You want one that is known to be open minded and pays attention to things like yeast overgrowth (candida) and other lesser known problems.
Is your stool greasy and yellowish in color? Thats how mine is, and i have had constant fatigue for the past 10 years. And if you want to know more about your brain feeling "clogged up" look up brain fog.
Basically you have something that is disrupting your digestive system, so you arent getting proper nutrients from your food and that is what is causing your fatigue and brain fog.
You just need to find the right doctor who will stop at nothing to help figure out the cause. A lot of them check for celiac, crohns, ulcerative colitis, and if it isnt any of those they just label it IBS and send you on your way with some bs medicine that probably wont do ****.
What I suggest first, is to start a food diary of everything you eat and drink and how you are feeling. After 2 weeks go back, look at the time you have an episode and see what you consumed a day or two prior. Maybe you can notice a type of food causing your trouble, eliminate it and you could be better. Most of the time these foods are gluten, grains, corn, soy, and dairy.
If after the 2 weeks you cant tell what food is causing the problem. then go on an elimination diet. Just eat things like grilled chicken (fresh, organic), steamed veggies, rice, red or sweet potatoes. Eat like this for a few weeks and see how you feel. If you feel better you can slowly add in other foods ONE AT A TIME i.e. eat a piece of bread and see if you have a reaction. Do that with whatever you normally eat and see if/when the problem comes back.
Another thought is to add a strong probiotic to your daily diet. You could have a flora imbalance, and this could help with that. Also, drink lots of water and try adding fiber to your diet to help bulk up your stool.