The day after my daughter who is 5 had a stomach bug and vomitted 6 times grabbed heart and said it felt like she was hit in heart with hammer- scared?

Irritated esophagus: The frequent vomiting likely caused a significant irritation from stomach acid in the lower end of the esophagus. Avoiding acidic or caffeine containing drinks & taking a Tums (calcium carbonate) may give temporary relief. A visit to confirm the process is a good idea.

Unlikely serious: The source of her pain was probably related to her vomiting and esophagus/stomach and not her heart. With the all the vomiting, she could have had a transient intense gas pain, or perhaps a muscle pull/cramp of her chest or abdominal muscles. Less likely would be a small tear in her esophagus (makes sure you see no blood if vomits again). Pain was probably one time event, if repeats, call doc.