Help! false blood test??

This month i didnt track anything, but i do know i ovulated tmdue to obvious cm. AF is a couple of days late, i have been breaking out on my forehead and my back which is so weird. I was having nausea at night whenever i lay down but since yesterday i had nausea on and off thoughout the day plus i would get a bit lightheaded! I have been getting both negative and faintly positive hpt. So I figured i would getmy  blood drawn and check what is wrong with me and wether im pregnant or not. I went to the ER to a general doctor. Got my blood drawn and the result were NOT pregnant. Could this be false?? The doctor said i might just have food poisoning, but i dont really believe that! Is it possible a blood test can be false??