Had 40g of allbran for breakfast as usually it's fine for me but I've just gone up to 15hope I come down so I can do the fasting at lunch
How far in advance did you bolus for your breakfast? If you're normally in range afterwards, it may just be one of those annoying things.
It does take a while to do a complete basal test so don't worry if you can't do it exactly as you planned.