Everybody was yelling 'straight right' but the punch to beat a southpaw is the left hook, and it was there for you the whole fight. When he was moving to his left (your right)you want to be hooking when he throws his left hand. When you threw the right hand, you needed to come back with the left hook. When you ducked under punches, the hook was there for you.
For a first time, you didn't seem to let a tall southpaw bother you much at all.