The nausea and tender breasts are really common symptoms for the first weeks of pregnancy. However, it's odd that the symptoms started a month ago. Either it's not pregnancy related; or if you have really long cycles, you could be pregnant; or if you have regular or shorter cycles, what you thought was your last period could have been just implantation bleeding (about a week after conception, when the zygote attaches itself to the wall of your uterus), and you actually are pregnant.
It is possible your pregnancy test was negative because, the way I understand it, the Human Gonadotropin that it measures takes a while to build up in your system after conception... I'm not sure, your body may not even start making it until implantation. Wait a couple more days & test again, first thing in the morning, with your very first urination that morning. That's when the HCG will be most concentrated.