Welcome
and sorry to hear you're having reaction troubles!
It's not unheard of for hives to come and go for days after a reaction (in our experience they will last anywhere from 3-4 days to a week). We always end up on steroids for several days after a reaction to keep the hives at bay. This time of year environmental allergies are often worse and you wore a jacket that likely had hay in/on it, so your body was 'primed' for a reaction which could have contributed to the lingering hives as well.
I agree with you that your two most likely culprits are the possible (probable?) shellfish contamination or the hazelnuts. I'd suspect a known allergen first, but it couldn't hurt to stop eating the hazelnuts until you can see an allergist and get tested for them.
FWIW, with a severe food allergy, it's not enough to just tell them "no prawns" because it is likely your food will still come into contact with the allergen during preparation - especially if it's steak and prawn night and the kitchen is handling a lot of your allergen. Prawn residue will be on the cooking surfaces, tongs and other utensils used to handle your food, cook's and server's hands, cloths used to wipe plate edges, cleaning cloths used to wipe your table, etc., etc., etc.