I have few questions to help me better assist you before I reply.
What breed is your dog? Is there a nasal discharge as well or is she only making noises that make her sound congested? If there is a discharge, is it clear or colored?
Is she sneezing?
Has she ever had either of these problems before? What dose of Benedryl are you giving and what does she weigh (approximately)?
Thanks and I will await your reply.
Most of the time dogs will exhibit dermatologic signs as a manifestation of their allergies, but with some dogs we will see respiratory signs as well or instead of. The signs are similar to what a person suffering from seasonal allergies goes through. There can be any combination of signs including sneezing, itchy or runny eyes or nose or congestion.
It sounds like this may be what your dog is going through since it is associated with her itchy skin. The dose of Benedryl you are using is correct and can be repeated 3 to 4 times per day. Not every antihistamine works as well for every dog or for every symptom. You could try switching to a different non-prescription antihistamine like chlorpheniramine (Chlor-Trimeton®) which may be a better decongestant for her and help her itch as well. Make sure there are no other ingredients for colds included. Her dose would be 10-20 mg twice a day.
If you start to notice nasal discharge that is cloudy or yellow/green or if she seems to be experiencing worsening congestion then I would recommend that you have your veterinarian examine her. It is less worrisome when the congestion is all in her nose versus her lower airways, but we don't want to ignore it if it seems to be getting worse or causing her distress.
Good luck with her and please let me know if you have any further questions.