The most common reason for a one-off episode of discomfort in the fistula during dialysis is likely to be infiltration around the puncture site ... sometimes some blood leaks out around the needle into the surrounding tissues - this is called infiltration - and can cause quite a bit discomfort. The needle can also be against the vein wall and cause some vibratory discomfort. Your mothers' clinic team are best placed to tell you the most likely cause in this specific instance. It would be wisest for you to seek their help and explanation, as they would be the most likely to know the specific circumstances of that particular dialysis session. Now and then, needles can just be in the wrong place, at the wrong angle, or have been pushed through the rear wall of the vein. Usually this is a once-off problem, but, again, the clinic are much more likely to be able to tell you which than I am. I would strongly advise you to work hard at building a trusting relationship with your mothers' team. This is always the best way to get advice and reassurance.