Implantation bleed?

I am so confused!! I am 4 days late, haven't felt pregnant at all until yesterday. I have done two negative tests n now my boobs are killing me! I totally feel pregnant (I've had 2 already) and last night I thought I came on but it was spotting. Could it be a late ovulation and a late implantation causing low hormones so negative tests? I really don't understand because I am so regular normally and everything points to pregnancy except the tests! Anyone else in the same boat?