The 33-year-old joined from Liverpool on Friday, signing a two-year contract.
"It is only two years ago that I scored against Scotland," Lambert told the Birmingham Mail.
"I scored 15 goals and then 13 goals for Southampton and obviously it is over a year since I did that but I have just got to get back to that now."
However, he refused to speak critically of his time at his boyhood club.
"I've got a lot of feelings, to be honest. I'm not going to get into it right now," he added.
"I feel as if I've got a point to prove to myself. That's always the biggest drive, yourself. And it will carry on being that way."