What Causes Loss Of Senses And Feeling Dizzy?

Hi Doc,I am a female/29.This has happened to me twice in last 1 year. I have felt being close to loosing my senses and falling. The first time it happened I started feeling dizzy, suddenly everything was blurred and I my ears turned deaf as if even in that busy shop non one was around. I immediately jumped out of the shop and fresh air helped me feel better. Gathered some strength after sitting for sometime then got some water and food and that helped. Though felt very weak. Same thing happened again when I was in train but as I started recognizing the symptoms I immediately jumped off the next station and sat down for sometime. I don't know why this happens. I have occasional low pressure but not perpetual.

Rgds
D