Yeah, doctors are geniuses about the nature of adaptation to exercise stress, aren't they? As far as the specifics of your situation are concerned,
1.) What specific bad thing is supposed to happen when you lift heavy, i.e. increase intraventricular pressure on the valve, and
2.) What is the valve material? Is it prone to failure or clot aggregation? I assume not, or they wouldn't have used it. But it might be useful to know the specifics.