My period was pretty regular up until june when i woke up one night with a very sharp pain in my pelvic area. Once i mentioned that to my doctor, he told me it was probably a cyst that busted. That month i was four days late, i was also late again in june by a couple days, and in august my period was a day late. I have a gyno appointment in early October for birth control to help me regulate my period but also i planned on taking another pregnacy test just in case after searching google for more info and it kept mentioning implantation bleeding.
The nausea and your period could be unrelated, or I know many women who get upset stomach before your period starts. Sounds as though you've been irregular the past few months. Stress and eating habits can change your cycle, or it could have been implantation bleeding and the pregnancy test you had at the doctor's was too early to tell. You may try another one, or you see if your period comes next month at the normal time. Good luck!