Insulin doesn't work after dialysis

Once in a while, insulin given after dialysis doesn't seem to work. My mom never takes insulin until dialysis is over as doing so drops her blood sugar levels during dialysis. She started dialysis earlier today at 6 am rather than her usual time at 11 am due to clinic's holiday schedule. She didn't take insulin and when she came back home at 11:30 am, her blood sugar was 170. We gave her 8 units of Novolog with lunch. She ate very little than what she usually eats, and 8 units is sufficient enough for what she ate. Suddenly at about 3:30 pm, she had a very bad headache accompanied with excessive thirst. When we tested her blood sugar, the meter read 'HI'. We gave her some water to drink and about 13 units of Novolog. We checked the blood sugar at 5:00 pm and the meter was still reading 'HI'. Again, we gave her 5 units of Novolog. At about 6 pm, finally the blood sugar came down to 360 and then at 7:30 pm, it was 160. We are still puzzled as to what caused the high blood glucose readings. This happens only on dialysis days. This has been happening for the 3rd time now and the doctors have no idea what is going on. They are saying they don't know what is going on. Has anyone experienced this before? I am very worried. Thank you.