tobacco artificially stimulates the endocrine system, including the thyroid which is the metabolic control system. An imbalance in the thyroid can cause exhaustion, brain fog, muscle aches, etc. I'd suggest getting your thyroid checked as well as the hormone panel mentioned above. Have your vitamin D and ferritin tested as well, as these can be low with thyroid problems and exacerbate symptoms. For thyroid tests, make sure you have Free T3 and Free T4 ran as well as a TSH. Don't let any doctor tell you that the TSH test is all you need. If you have questions or need more info feel free to PM me.
I understand the need to feel better and going back to
tobacco. I quit cold turkey for 7 months and started smoking again, both from the need and from feeling like garbage because my thyroid was crashing. Rather than going back to cigarettes though, you might try using swedish snus. They have a decade of studies behind them proving that they are many times less hazardous than smoking. There's a subforum here for smokeless if you're interested in learning more.
Best wishes to you, whichever road you travel.
Edited to add that pamdis' post above is also a possibility and all things should be investigated. Good luck!