My son is becoming a brat

I don't know what's going on. I hate using the word brat but I don't know what else to call it. He eats perfectly and is learning everything so well. His health is great and over all a great baby.... But when he is put down or in his high chair or walker he throws a straight up tantrum. I can't put clothes on him or a diaper or give him a bath without a tantrum with it. We are expecting our next baby in 30 days (hallelujah!) And I think he knows what's going on. I haven't taken any attention away from him at all and love playing with him. I've been nothing but patient with him though. I just don't condone to his fits and I won't let family condone to it either. Not unless it's over something bad like and he is crying bloody murder. Am I the only one?