What Causes A Scanty Menstrual Period,enlarged Breasts And Nausea?

I have a question the past two months i have had very short periods they only last 2 days, and me and my fiance have had unprotected sex for 4 months now, my breasts have grown, i sometimes feel nasueas and my appitie has increased. I have taken a fair amount of home pregnancy test and all negarive. Could i be pregnant or is something wrong with my body?