Johnson confronted the Colombian during the first half of the goalless draw at St Andrew's after being caught by the forward's arm.
Rodallega reacted by twice pushing his forehead towards Johnson's chin, with both players picking up a yellow card from referee Mark Clattenburg for the incident.
And Johnson claimed it was not the first time Rodallega had caught him in such a manner.
Johnson told the Birmingham Mail: "That's the only reason I got the hump. He did me last year and cut me. It was exactly the same challenge again. I think I was within my rights to have a word with him.
"The ref's come in and it was a harsh booking for me, to be honest. If it happens on the street, then you go and have a word with whoever did it."