It means youre posting in the right place. When you go from sitting standing or lying down positions your body overcompensates for shifts in blood pressure.
Its by no means serious, you just need to take your meds as directed, stay hydrated, eat right, and stand up slower
Mitral valve prolapse is a different unrelated issue entirely.. That can be serious depending on just how prolapsed it is.. Sometimes its minor requiring no treatment, other times its serious requiring a valve replacement.
Good news is.. If its serious then its super obvious on echo... This isnt something a doctor would "miss". You can take his assessment of it to the bank.