Hi, I am an American Board Certified Neurologist with vast experience in diagnosing and treating neurological & all health conditions including food & fish allergies and I am very glad to help you with your health concerns.
You are doing the right thing by taking an antihistaminic medication like Benadryl and it can help with most of the mild cases of shellfish allergies.
More severe attacks will present with breathing difficulties or swallowing issues etc you are not having them so this looks like a mild allergic reaction
However you need to observe further, for the next several hours.
Most of the shell fish allergy symptoms start to occur within few minutes and most of the patients will have full blown symptoms within couple of hours. Very rarely symptoms may start occurring about 5 hours or later, but when symptoms occur so late they are almost always mild in nature
But the symptoms occurring soon after eating the fish can be severe due to severe allergic reaction and this will manifest as difficulty with breathing and swallowing etc and if this happens then you need to visit the ER immediately so that you can receive emergency drugs like Epinephrine shot & Steroid shot to resolve the allergic reaction
Based on the information provided by you in your question it sounds like you are not having any such major symptoms like breathing or swallowing difficulties etc so this appears mild allergic reaction and Benadryl will help too and this should resolve soon. But observation for the next few hours is important.
How are you overall feeling now?
Is any of your symptom seems to be worsening?
No difficulty with breathing or swallowing agree?