What Causes Slow Heart Rate And Severe Nausea During Menstruation?

I m on my period and I m experiencing a slow heart rate. I feel like I m going to faint and I have severe nausea. I had to get someone to carry me to a place to lay down. I ve had this problem only on my first day of every other period. I have regular periods that may or may not be heavy. I m not on any medicine and, I m not anemic. i went to several doctors about the problem and they tested my blood with everything coming out as normal. Please, tell me what to do