Negative test but think I maybe pregnant.

I am 2 1/2 weeks late for my period. I have taken 4 home pregnancy tests all after a week late for my period. And one at the dr office this past Wednesday (2 weeks late). They have all said negative. The day I went to the dr I had some spotting, which I thought was the start of my period. But disappeared that same morning. I am now having back pain (siatic nerve just like my first pregnancy), upset stomach with diarrhia for the last 3 days, and exhausted. Is it possible to get a negative test and be pregnant.