Can after a month of congestion be viral cold or pet allergies 6 month old baby?

Maybe: A month of congestion could be from an infection like RSV or could be due to reflux or a bacterial infection. Food allergies could be a cause, but not pet allergies at 6 months of age. See the doctor for the answer.

Pet allergy: It is unlikely for a viral cold to cause symptoms lasting more than 2 weeks. However sinus infection, which is often the result of a cold, or allergy can cause congestion lasting months or longer.

Look to environment: Since the nasal passages are lined with sensitive tissue, it can and will swell in response to respiratory irritants and a variety of infectious agents. Congestion is most noticeable when infants have a small head/nose/nasal passages and are not yet mouth breathing. This improves with head growth, but any child will have recurrent nasal congestion when conditions set it off.