Heater blows hot on 90 only

[spriddle]

I'm new here and actually posted this in the GMC section but not really getting any ideas what the problem is there so I'm posting it here in hopes someone else has ran across this problem in the Chevy section.

I flushed the entire cooling system on my 05 GMC Z71: radiator and heater core (backflushed by removing the heater hoses and taking a garden hose and flushing thoroughly). Now my heater will only blow hot if turned up to 90. You can go down one notch to 89 and it blows cool. It does have separate climate control. I have no idea what in the world could've caused this to happen simply by flushing the system. I would think maybe the heater core was stopped up but both heater hoses are hot and the heater does blow hot but only on the 90 setting. Any help or ideas are appreciated. Thanks.