Posture springs to mind for me, if have to keep still and are not in the most comfortable position and when you are vomiting a lot, maybe even increased tension in and stress on your body from the stress of the attack, trying to stay so still and, again, all the vomiting.
Also if you have full on drop attacks which throw you to the floor.
Are you flat to the floor and stuck there during the worst phase, or can you move slightly up to vomit in a bowl/the toilet, depending on how severe the vertigo is for you? That sprang to mind for me in the sense of irritating your lower back, being tensed up over a bowl or something, especially if a prolonged attack and a lot of vomiting.
How severe are your attacks? Would you be able to keep down any pre-emptive painkillers/anti-inflammatories just after an attack to see if that helps? Last thing you want after a bad attack, to be in agony as well.