i have missed two periods and have taken pregnancy tests that have all come

[Sam]

back negative? when this all first started i had missed a period in march of 2009, i took six preggo tests and they all came back negative, i went to my local physician and he had me take a pregnancy test it came back negative also, he told me that sometimes woman just dont menstrate. the next month (april) i had a normal period. then in may i missed it again. it is now june and still no period, i have taken four tests they all say negative, i have an appt set up next week for the doctor for a blood test to find out for sure, but i am really concerned and extremely worried, how accurate are the tests and should i be concerned that i am pregnant. any help that anyone can give me is greatly appreciated thank you very much