Talk to Eye Specialist on I Get Flashes In My Eyes

I get flashes in my eyes. It happened after almost 6 months and happens after long time. It starts with few dots of flashes and then spread all over so that barely i can see anything and only i can see flashing lights. It lasts for 20 to 30 minutes and then all is fine but then i get headache for which i need to take disprin or panadol for relief. I visited eye specialist but he said there is nothing to worry but i don't understand why it happens and what is cure for this problem?