Need help

Af should start dec 4 but last friday i started to spot it started brown and a little red only when i wiped today it seems like if my period started. I have a lot of lower back pain kinda like cramping i thought that maybe it could be implantation now im not to sure i took a pregnancy test this morning it was negative. Af has always been on time or maybe two days early but not a week what should i do?