What Causes Fainting Frequently?

I faint quite frequently (about 3 times a year) and I always know just before I am going to because of blurry vision and popped ears. However I don't know what triggers it. When I faint Im usually out for around 10-20 seconds but sometimes my eyes stay open or are fluttering (once been told I looked like I was having a fit). When I come round I normally feel sick. Im quite worried and hope you have some answers.