Could it be that I need to drink more?

Today I was waiting to get sth for breakfast at a counter and I felt I was going to faint! My blood pressure probably went down or something and I felt my ears blocking and I started seeing all black and sweating! I tried telling the lady at Starbucks that I was feeling sick and she just said -oh you need more time...and turned around! I got really scared that if something had happened to nobody was going to help me! Do you think not drinking enough water could be the cause? Anybody else had a low pressure episode?