Brief Answer:
Cardiac cause, 
Orthostatic Hypotension or anemia
Detailed Answer:
Hi,
I went through your query and understood your concerns.
You are suffering from mild low BP (below 100/60 is hypotension).
With the history of mitral and atrial valve regurgetation I would like to advise you  for cardiac evaluation once as the direct cause for low BP may be in heart. In 
aortic regurgitation there is rapid decrease in blood pressure during diastole due to blood returning to heart. So if condition worsens the treatment also changes. So needs cardiac evaluation once.
The raise in pulse rate may be due to 
reflex tachycardia due to fall in BP for maintaining blood supply to vital organs. Metaprilol controls BP by reducing 
heart rate, but since you have low BP higher dose of 
metoprolol may worsen the condition. 
Orthostatic Hypotension (OH) or anemia may also possible.
For OH the test measures the person's blood pressure while seated or reclining at rest, and again upon standing up. A sudden, significant fall in blood pressure (>20 mmHg) between 2 and 5 minutes after standing from the supine position indicates orthostatic hypotension. In addition, the heart rate should also be measured for both positions. This and certain other clinical examination findings can help your physician to confirm the diagnosis.
The treatment includes lifestyle changes in milder cases and medications in severe cases.
Even in severe 
anaemia or due to blood loss the BP will be low with raised pulse rate. So you need to rule out anemia with simple 
blood test (complete hemogram).
So please do not worry and do get yourself evaluated once.
Hope this answers your query. If you have additional questions or follow up queries then please do not hesitate in writing to us. I will be happy to answer your queries.
Wishing you good health.
Regards