Yesterday I Had A Bout Of Fainting, Nearly Lost Consciousness

Yesterday I had a bout of fainting, nearly lost consciousness and ended up nauseous and vomiting for nearly 4 hours. Finally was able to hold down a little chicken broth and ginger-ale. This is the second time this has happened - first time March 14th, then April 19th. I am a 75 year old woman with an artificial aortic valve and after the first attack had the heart checked out. The cardiologist found nothing. Also had a CBC with my primary physician and nothing turned up. I don't know what to eat, as this has happened after a meal both times.