Could I have cardiomyopathy after the stomach flu? How would I know?

It could happen. But your chances are overwhelmingly in favor of doing fine. A lot of gastro-intestinal viruses can cause cardiomyopathy but it is usually a combination of the bug and the patient's genetic predisposition of developing the disease. Symptoms can be very non-specific like palpitations, shortness of breath and increased fatigueability. The diagnosis can be made with an echocardiogram. See your doc.
Yes, why? Dilated cardiomyopathy can be a result of a viral infection. Symptoms often occur after the initial viral symptoms (such as the flu) have resolved. I assume you ask because you think you may have cardiomyopathy. You should see your doctor if you have not fully recovered from your recent illness.