Checking with your doctor and doing tests is the most important and the best approach. Hopefully, the tests will show clean bill of health or something easy to fix.
"
I thought that I wasn't training enough, but I realized that wasn't the case. More riding resulted in more fatigue."
This might indicate the possibility of overtraining:
"
Here are nine signs of overtraining to look out for:- Decreased performance. ...
- Increased perceived effort during workouts. ...
- Excessive fatigue. ...
- Agitation and moodiness. ...
- Insomnia or restless sleep. ...
- Loss of appetite. ...
- Chronic or nagging injuries. ...
- Metabolic imbalances.
"