What Causes Dizziness And Tingling In Feet?

I am 35, weigh 9st 12 and am 5ft 7, I have been suffering from dizziness for about 5 years, i get dizzy sitting down at work, in lifts, when shopping, there is no one place or time that I can pinpoint, i also get tingling in my feet when this happpens. I have been to my GP and had a test where by they strapped me to a bed and i stayed in the same position for 30mins, nothing happened to by blood pressure/pulse. When i feel dizzy i usually put on my glasses and i start to feel better. please can you suggest the next steps as it is not going away. It usually seems worse 2 weeks before my period is due. Thanks