Can an allergy to the mixture appear after a month of taking it? The…

Can an allergy to the mixture appear after a month of taking it? The doctor says that he can't, but I see that the child's condition is extremely strange. She has been breaking out (the doctor says that either I have something eaten (with us). Today her eyelids swelled up (they wrote to me that this is because she cried or we need to ventilate more. But we always ventilate everything) the general state is weak I am sure that this is on the mixture, but why does the doctor say the opposite?? And tell me, how did you switch from formula to formula? And if another one doesn't work, again jump to the third one?