Low blood pressure or hypotension can be due to conditions that reduce the volume of blood, reduce cardiac output and certain medications. Dehydration, bleeding, shock, heart diseases, vasovagal reaction, postural, adrenal insufficiency, septicemia, and anaphylaxis are differentials that you may consider. Take care and do keep us posted.
Internal bleeding, aortic aneurysm is another possible cause, dehydration, pancreatitis, pulmonary embolism. Use of some drugs. Have them check all of those.