You seem to be suffering from episodes of hypoglycemia
,seen at this stage of pregnancy
with some strain. You should also exclude transient hypotension
. These things happen as you strain for some time like when you stand. The blood flow from the extremities is reduced owing to pressure on the veins due to the enlarged uterus
. Try eating small meals frequently and also try sitting in different postures. Please take further advice from your doctor. Wish you safe motherhood.