Is this implantation or early period? HeLLPPp

So I had unprotected sex 9 days ago and would be due for my period around the 15th August I think, normally I am very regular. On Sunday evening I started bleeding and had to wear a really thin liner,but I only noticed it when I wiped. It was really dark brown and not like a normal period at all. My periods are normally light but not this light and I have never started a period in the evening before! My breasts have veins in them which is how I've noticed I've been pregnant before. Do you think this could be inplantation or would it only last a few hours?