My 3 mo. old can't breathe through nose?

My daughter is 3 months old and and has done fine when eating til 2 & 1/2 weeks ago, and now when she eats she acts like she can't breathe out her nose, so she gets frustrated, and fussy, and she isn't eating very well now, and i've suctioned her nose with saline drops, i put her in the bathroom with the shower running for the steam, we have done humidifiers, and i've taken her to her doctor just the other day, and nothing seems to be helping her any, but my son has an allergy to cats, ans we have one cat that catches mice, but my son doesn't mess with, so could she be like my son and have an allergy to cats? But, can anyone help me to help my daughter?