False negative??

How do false negatives happen?? I missed my period (supposed to be here the 2nd of July) but did have spotting a little on the 3rd but only a very little amount (one bathroom break worth and it was pink) and since then I haven't bled At all. I took a first response and got a faint line when I looked about 10 minutes after I took it. But then I took ept ones and they were terrible. I could berly see the control line.. I can't go to the doc right now because my health insurance is ended until I get a job with health insurance..I have been throwing up a lot. My sense of smell deffinitly has gotten better .. I can smell everything! I'm always nauseous. I have mood swings like crazy.. I'm bloated . My last period was June 2-7....Please some advice?