It's called orthostatic hypotension. It is caused by a rapid drop in blood pressure and results in a momentarily inadequate flow of blood to your brain. You most likely experience it when standing after sitting or lying down. It's a symptom of PD and can also be caused by some medications. It is treatable, so mention it to your doctor(s). It's common in Parkinson's patients, so I'm quite surprised your doctors didn't know.

When getting out of bed, sit on the edge for a short while before standing. When sitting then standing, don't wander far from the chair in case you need it to steady yourself.

My mother has PD & gets dizzy sometimes.
She takes Meclizine when she gets dizzy & this works for her.

I'm lirheaded every day...very little safe balance. My life is different now.

My mother has PD & gets dizzy sometimes so she takes med for it. Drinking a lot of water is important to prevent dehydration which causes dizziness.
Also getting up too fast from a lying position.
Aside from PD info...inner ear can be the source.

It can also be caused by low blood pressure.