Shellfish especially,and fish are the most common cause of gastric
problems , wherever they are taken, travelling, home hotel or not.
They can cause the most violent and quick reactions.
Agree with all the comments above.
I would always think it advisable to seek medical assistance if symptoms continue after 24 hrs.
Re-hydration salts / sachets are easy and cheap always to take, as would a multivitamin dispersible tablet. I manage medical services and i always take some basics with me including this.
It would be frightening to be alone in a place or country where help is not so easy to obtain, you are isolated georaphically, or language problems.
I think any travel these days, some basic First Aid items, that YOU know you can take safely, tried and tested before, would be very wise. These would include simple things like rehydration salts or vitamins, pain relief /anti inflammatory/ anti-sickness/ antistermine…
Regard these as First Aid, not just the kit plasters and creams.
Never gorget your EHIC care within the EU, it will support in situations such as this.
Sorry to hear, and I hope all is well now.
It sounds as though the hotel were up-front, open, and have clearly sympathised, and you have been fairly treated.