Fainting

Has anybody fainted throughout their pregnancy? I am 13 weeks today and yesterday at school I fainted and hit my head pretty hard. I had no memory of what happened and I was out for about 15minutes I guess. Has anybody had this happen before? I went to the ER because I kept repeating things I guess and my baby is perfectly fine thank goodness. I am just feeling unwell with horrible constant throbbing headache. Just wondering if anyone else has experienced something like this!