Everyday I feel so fatigued

And if I stand or sit up for too long I start to feel really lightheaded and I only feel better once I’m laying flat. Idk if it’s low blood pressure bc I have been getting high readings at times at the docs office. I passed my glucose test so I don’t think it’s blood sugar.

Anyone have experience with this?

This happens every day starting in the morning after I eat. It’s worse then and gets a little better throughout the day. I didn’t have much sugar. Cheerios with milk and water to drink.