I Have Gotten Sick Smelling Food That I Always Ate.

I have gotten sick smelling food that I always ate. Spotted for the last 4 months, but no real period. and really tired, but have tested negetive. I was told that a low HCG would cause negetive readings and my 3 kids that I have never tested until I was at least 5 months could it be possible to be pregnant and not test positive til 5 months again?