Hi Seixomnia,
When lying on your back you are putting more pressure on the lower half of the vaginal entrance, so you would feel a more intense stretching sensation there.
When vertical, the dilator is not pushing on that band of muscles as much.
I would continue to dilate in both positions, as you are doing great work both ways. And being able to insert something in different positions is important.
I think dilating on your back really helps with making sure you target the muscles that surround the lower portion of the vaginal entrance.
Hope this helps.
Melissa