What Would Cause Delay In Periods?

i was 12 days late on my period and i started bleeding 2 days ago the blood was burnt red and a light flow. and today it's very light red and practicably done. i have taken pregnancy test every 4-5 days and there all negative i took one this morning and its negative. can i still be pregnant? is this implantation bleeding? if it is when can i test again to get a posative and if im not why was i so late?