Hi, Welcome to Health care magic forum.
It appears that you have a polyp, long back, and now you have the swelling of the trabiculae, in the
nasal cavity. They swell temporarily due to
allergy.
As you are allergic to the shampoo, you are getting the obstruction, while taking the shower.
When ever you are having the allergic product like food, soap, shampoo, cosmetics, etc and subsides with the decongestant drops. Because it is different disease.
Of course if you want you can consult another ENT surgeon, for second openion.
Avoid taking fruits, juices, ice water, drinks, and icecream during winter season.
Wishing for a quick and complete recovery. Thank you.