please help

I just wonder if anyone could help I was due my periods today but got them on the 17th my periods only last 4 days but I still have them and it doesn't feel like a period it's been very light bleeding no blood in my urine or on my pad just light pink when I wipe and I'm really not sure what it could be not had cramp but tummy feels really weird and back is so sore was thinking it might be implantation bleeding and I took pregnancy test yesterday but it was negative really done no why this is happning