What is this pain?

Kay • Baby #1 September 2015
So I don't think im pregnant . I've darts having sore breasts 4dpo so I though, "Okay, guess I'm not pregnant." And today is 8dpo and I am having extremely sharp pain in my lower abdomin. Very painful and it started last night. But that would be too soon for implantation and my perio isn't due till the 3rd. What could it be?