Brief Answer:
This can be explained with Orthostatic hypotension.

Detailed Answer:
Hello and thanks for asking.
I read carefully your question and understood your concern.

If your symproms are not related to low blood sugar than in my opinion the cause can be Orthostatic hypotension.

This condition — also called postural hypotension — is a form of low blood pressure that happens when you stand up from sitting or lying down. Orthostatic hypotension can make you feel dizzy or lightheaded, and maybe even faint.

Mild orthostatic hypotension often doesn't need treatment.
Many people occasionally feel dizzy,nausea or lightheaded after standing(just like you).Don't worry,Usually this is not cause for concern.

It's important to see your doctor if you experience frequent symptoms of orthostatic hypotension because they sometimes can point to more-serious problems.

The goal in evaluating orthostatic hypotension, as with all forms of low blood pressure, is to find the underlying cause.

For this reason i sugget to consult your doctor because it is necessary to do some test like blood pressure monitoring,ekg,echocardiogram,blood test to check for anemia etc.
These test will determine if orthostatic hypotension is the cause of your symptoms and what can be the cause of orthostatic hypotension.(Let me know your test results)

Hope i have clarified your doubts.
If you have other questions,feel free and ask.
Wish you good health.

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