??
Let me enlighten you.
Aizen's first conclusion was that he did that because he couldn't feel any spiritual pressure from Ichigo at all. At first he thought it was because he let go of it. Right after Ichigo became one with Getsuga and before he threw his "final" attack Aizen received a second thought. He started reminiscing that humans and mere soul reapers couldn't feel Aizen at all unless he lowered his own power to a level of their awareness. He then realised that Ichigo had so much power that it had to be that, in a despair of denial and complexity he rushed Ichigo yelling "It can't be!" I just re-watched it to make sure since I could be wrong. Yes, Ichigo had so much power that Aizen's abilities couldn't even sense him, Ichigo never gave up his powers BEFORE he made FGT attack (which Aizen survived but Urahara had the better of him
)
You are welcome to re-watch as I did or read the chapters.