Implantation only 5 dpo?

Hi Ladies! I've been "feeling" pregnant since we DYD! I know a looooot of it is in the mind! Today is 5 DPO and my breasts are really sore and big (hurt to walk) and I have a mild cramp...went to the toilet just now and saw a very smal brownish stain on my underwear and almost nothing on the toilet paper! Could it be implantation??? Or just AF arriving 8 days ahead of time?