He said: "I honestly didn't know the rules and I will have to get the rulebook out later tonight but other than that we have to be happy with what has happened.
"I was a bit gutted to leave the lads with 10 men and it could have been a different story but I am just happy we got three points and this is a hard place to come to get a result.
"I am feeling strange at the moment but these things happen in football."
Cahill added: "I suppose if that's the rule then that's the rule and I have to deal with it.
"I didn't actually take my shirt off. The goals I scored with Millwall last season I always put my shirt over my head and the FA Cup semi-final I took my shirt off.
"It just baffles me in a way but it one of those things that if you don't know, you don't know."