I just ate days-old fish and hours later i felt nauseous and i keep vomiting but i dont have diarrhea and no stomach pain at all. i thought its an acid reflux but i just had an antacid and im still vomiting from time to time. is this stomach flu?

You likely had an acute food poisoning and I guess you had vomited all the preformed toxins in your stomach before significant amount of toxins got to your intestines. You still feel nauseous and can still vomit for several hours. It is tough to rule out an acute viral illness (stomach flu) but food poisoning is more consistent with your clinical scenario.

The timing of eating the fish and your GI symptoms seem connected. Diarrhea can come later. There are many other reasons for your symptoms such as a virus. My experience with the acidity in this scenario is that patients have underlying long-term gastritis, that they may not feel, that is triggered by certain foods such as spoiled foods.