He was referring to his sickness! Itachi was already sick and he had to use susanoo!It was probably a side effect of overusing his Mangekyou Sharingan.
Sasuke stated that every body cell hurt while using Susano'o, by that time, Sasuke only had a humanoid incomplete skeleton version.
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Itachi already by that point mastered his Susano'o to his limitations.
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Do you have any scans? This would be very useful information.It seemed like asthma in the flashbacks. Stress and depression due induce that.
I will post them tomorrow.Do you have any scans? This would be very useful information.