HELP! (medyo spg)

2 years na kami ng boyfriend ko pero recently lang na may nangyari samin.
We have been using the pull-out method for 4 months and usually 2-3days ang pinaka delay ng period ko. For 28day cycle its supposed to come this december 15 but up until december 22 wala pa rin dumadating and I noticed some brownish something sa underwear ko since december 21, though I'm not sure if it's my discharge or what. I also feel the usual period cramps which is pain on the both sides of my waist. I also took pregnancy test december 22 (8th day of my delay period) and it turns out to be negative. formal items in navy blue
For girls out there did you had the same situation? Please help me. Is this normal or am I pregnant?
AC
20**
UP