There are other things that can give a positive test, such as an overian cist or some other quite serious medical conditions...
Overian cists can also cause nautiousness, and stuff with your periods, as well as give positive home tests, but negative blood tests... they arnt that harmfull (i know, mum had one not that long ago), but can become dangerous if left untreated.
Also some medications can effect home pregnancy tests, so if your on any perscription medication, you may want to check the net to see if it interfeers...
theres also a chance that your doctor stuffed up or got your sample mixed up (they mixed up my sample last time i was in the emergency room, i had appendicitus, and they told me to go home cos the tests didnt show anything, consequently we told them to run the tests again, and i had my appendix out the next day, but that irrelivant

) and they may have given you someone elses results by accident or something...
The general gist of this long ramble is you should go back to your doctor, get another set of tests done, and if they still come back negative get them to check for other conditions, or issues with medications.
Goodluck