Start of period or implantation?

My period is due sometime between today and the 21st. I've been having light, light, light cramping (like just barely noticeable) for a couple of days and today I noticed this 
It looks lighter in person. I've never had this before a period so I'm slightly confused. I think it could be the start of AF. The only other symptom I have is about my boobs. They're very sore and firm. Kind of have more blue veins but I noticed that about a week ago. I'm not sure how many dpo I am because I haven't taken any opks.