I'd start with asking your VMware Admin to check the memory usage statistics for the physical host of your VM. The reason I'd start here is because its very quick to check on the VMs from a high level where as memory investigation within the VM itself and SQL Server will require more time.
Confirm you're VM isn't having pressure from it's neighbors and then drill down into it.
Here are the esxtop commands. He or she would just have to look at "MEMCTL" (MB) to tell if there is ballooning taking place.