Spotting 2 weeks after missed period

Ok so yesterday made 2 weeks of missed period. I started spotting (only when I wipe). It very light pink. Today I'm still spotting only when I wipe and it's still very light pink. I've taken a pregnancy test since 12 dpo EVERY morning and sometimes twice a day. Every result I've gotten has been negetive. I've been having tender breast, heartburn and throbbing headaches. Does anyone know why this is happening? Any advice would be appreciated.