What Causes Nausea,scanty Periods,heartburn And Fatigue?

I have been nauseated since the beginning of the month (April), vomiting on and off since the 2nd week of the month. My period was due April 9th, which came on time, but not as heavy.. medium flow for 2 days then light flow for 4 days. I took a First Response pregnancy test and got an instant white line in the result area. I have also been experiencing mild heartburn, headaches and fatigue. My next period is due the 7th of May when should I test again. If I am not pregnant, what could be causing these symptoms.. nothing in my life has changed, no added stress etc. Thank you