If it's related to a food intolerance/sensitivity, I'd recommend trying zymptom.ai. You basically log your foods each day and the intensity of your symptoms. The AI breaks down the foods into ingredients and categorises them into common food intolerances (gluten, dairy, caffeine, high FODMAP, high glycemic index, etc.) and finds any patterns between your food and your symptoms (if there are any)
Hi, possibly hypoglycemia. You can buy a blood glucose meter for ~$25 and test when you have an episode. It can be reactive type or the fasting type.
shadowmed mentioned anemia, in most states you can get a CBC test done without needing a doctor, which will reveal any anemia.
Also, narcolepsy comes to mind. Then you can look at a Differential Diagnosis (aka DDx) of other possibles, like this: https://emedicine.medscape.com/article/1188433-differential
Good luck.
I think you are right to see a doctor so that they can investigate if there is anything in your blood that explains this. Do you sleep ok at night? I understand that sleep apnea can cause people to feel extremely tired during the day, even though they think they’ve slept ok. Anaemia can make you tired too, as can asthma. I’d just be guessing though because without proper investigations I doubt you’ll get the right answers. Good luck with finding the cause and please let us know how you get on.