Lamah, 25, claimed he quickly told referee Lee Mason of the mistake but admitted he could do little but accept their incorrect decision.
He said: "My goal should have stood. I was onside and the referee was wrong.
"I said to him I was onside and that it came off the defender, the goalkeeper and then me.
"Normally it's a goal, but it was his decision and I had to accept it.
"A draw would have been a very good point for us because West Brom are so strong at home and it's disappointing to lose - but this is football."