Headaches :(

was hoping for a little imput ladies.. i will be entering my 24th week right away, and for the last like 2 months ive been getting minor headaches.. thing is the are basically every second day.. like clock work. they are not horrible and i dont take anything for them, but by the end of the day i am soooo drained.  i talked to doctor and he didnt really seem to listen and misunderstood what i was saying, mentioned a sinus infection and gave me a persciption for a nasal spray. thing is... im not sick, its been like 2 months and i know has nothing to do with a cold or anything.. so ive been trying to narrow down what it could be caused from.. i eat very heathly and alot. lol i drink gallons of water and sleep lots latley.. these headaches dont start till about 2 in the afternoon.  and the last 3 days ive been experimenting .... wednesday i forgot to take prenatal vit.. no headache. thursday took vit.. headache... today i purposly did not take vit.. and wala no headache.. what do u ladies think? any ideas or input?  just getting so frusterated and sick of feeling bad.. any advice would be appreciated.. thanks.

[wasabic]

i had a lot of migraines but the doc said is from the blood flow being higher wile u pregnant just to take an tynelon when i cant take the pain , i also get some headaches from the allergies but they dont get horrible like the migraines so i just wait in till they pass case my doc dont want me take pills for them anyway hope it helps and good luck finding the reason and get rid  off them !