Questions about possible pregnancy.?

I had 1 positive pregnancy test 2 weeks ago, had very very light bleeding so I called and made an appointment for an ultra sound. My appt is Dec. 27, but my Dr. had asked that I take another test to see what it says after the light bleeding. There was no clots or heavy cramps (light cramping but only once). But right now (I was suppose to get my period Dec. 7th) I am so bloated, always having to go to the bathroom, constipated, my hurt, really dizzy and nauseated most of the day. I took another test today and it was negative. Any thoughts? Do you think it could have been a false positive to begin with? I looked at WebMD and it showed my top answer could be a tubal pregnancy. I'm confused and really scared that something might be wrong, and wondering why I still have all these symptoms...