The Blues were soundly beaten at the KC Stadium after conceding a goal within the first minute.
"It is a tough place to come and when you are 30 seconds in and 1-0 down to quite a simple goal it is a tough one," Clark told Sky Sports News.
"From there it went from bad to worse and we were 3-0 down in half an hour. They were not great goals from our point of view - awful in fact."
A 13th defeat of the season has seen Birmingham slip down to 17th place in the table.