@ysmina-- There are anti-diarrhea medications at the pharmacy that you can take for diarrhea. It might also help to take an antihistamine allergy medication.
Right now, it sounds like you have indigestion from the oyster due to sensitivity or mild allergy. But if you develop more severe symptoms like hives, swelling and difficulty breathing, you need to go to the hospital right away. Anaphylaxis is very dangerous and requires medications like epinephrine.
I'm allergic to shellfish as well, my allergy is very severe so I carry an epinephrine pen with me at all times in case I accidentally consume shellfish traces in other foods.