Is it possible that fetus is allergic to fish? I usually love it but not when pregnant

No. The immune system doesn't develop to the point of being able to produce allergic antibodies until about six months after birth. The hormone changes of pregnancy can change your immune system; it is not uncommon for pregnant women to develop different allergic symptoms or intolerances while pregnant. Many of these, depending on the cause and severity, disappear after delivery.
No. Allergic reactions in the fetus have not been described american academy of pediatrics guidelines do not recommend restriction of a pregnat woman's diet to prevent allergies. Pregnant women develop strong food likes and dislikes unrelated to food allergy and these tend to resolve after giving birth.