What Causes A Missed Period With Negative HPTs?

I have missed my period by 3 weeks now. I have take 4 pregnancy test all of which have come back negative, I have spoken to my GP and he said there isnt anything he can do until i havent had a period for 4-6 months! I have a history of miscarrages having already had 4 a one healthy little boy. I am worried that there might be something wrong and i dont know what to do about it. Can you give me any advice?