The same thing happened to me two weeks ago and I admitted myself to the hospital just in case. Normal BP, normal blood glucose, normal thyroid function etc. Turned out I had low blood count and was put on additional iron supplements. Seemed to have done the trick for me.
Oh dear, fainting doesn't seem normal in pregnancy. You might have low blood pressure, low blood sugar, dehydration, hunger, etc... Please do consult your gynae and wishing you well.