I totally agree with Spade22
Tests are accurate about 2 weeks after conception therefore, you wouldnt have been more than 14 days pregnant when you took your last test.
I suggest you go to the doctor - feeling nauseous for 4 months is not normal. They can also run a blood test to rule out pregnancy but it doesnt sound like you are (and if you are, you wouldnt be more than a few weeks).