fainting

I'm 18.5 weeks pregnant. For work I have to stand in one place for a long time without sitting. It's also kept somewhat warm. This is the second week in a row I have fainted at work. Yesterday I hit my face on the concrete floor! I saw my OBGYN & a cardiologist & they both agreed I'm dehydrated. I am drinking lots of water & taking the rest of the week off but I'm terrified this happens again on Monday! Any body else experience this in pregnancy?