Here's the situation:

My opponent plays miracle, I play Death and taxes.
My opponent has a Top in play.
Turn 2 I play Thalia.
Turn 3, my opponent plays counterbalance pays only 2 mana (Yes i was very tired) and keeps the possibility to run top.
My turn 3, I try to play a non creature spell (Sword of fire and ice), my opponent reminds me that I have to pay 1 more because of Thalia.

It was a minor tournament so I did not say too much (In reallity I should probably have hit the guy in the face for being such an ass. or not. violence is bad).
could it be considered as cheating if I call a judge?