Not enough lubricant. Try rotating the dilator gently to remove it and add more lubricant A vacuum, like ketchup stuck in the bottle. Try changing positions. Rolling onto your side or standing up may help. Everybody is different, and there is no way of knowing how much pain you may feel.
Some people say it's more uncomfortable than painful.
As always, your mileage may vary. The tighter those muscles are, the more difficult it can be. Adequate lubrication is essential. Use an applicator to place lubricant into the vagina. Don't dilate with a full bladder or bowel.