What Causes Appetite Loss While On Bystolic?

I have been taking bystolic (2.5mg) for approximately 6 months and my loss of appetite has become so profound that I m becoming very weak. I also went from being able to exercise for 30 minutes every day to only 2 minutes before having to lay right down. That was before I lost my appetite. I should also mention that I have an uncorrected Secundum atrial septal defect with moderate valve regurgiations. The last echo I had was last July and I was just wondering if taking bystolic could be responsible for any of these recent manifestations. What do you think, any thoughts would be greatly appreciated.