Beginning of period or pregnancy?

Tatyana • 03/23/17👦🏽 04/03/18👧🏽

So I usually start my period on the 16th of every month, but yesterday I was having bad lower back & stomach cramping. Earlier today when I wiped down there I had really light pink blood so I thought I started my period early. Right now when I wiped It was really dark brown & stringy. I’ve never had this before. I’m still cramping in my lower back. Could it be my period or implantation? Has this ever happened to you?