I Have Been Feeling Lightheaded For The Past Three Weeks

I have been feeling lightheaded for the past three weeks on a daily basis. sometimes i feel like i'm going to faint. i have to hold onto things. what could be the problem. my doctor has done blood work - checked glucose, iron , etc. and everything comes back normal. my sugar level (96) was just a little low. my cat scan was normal. i am getting an mri soon.