I tested a few days before I was due, I think it was 11 dpo for my first pregnancy and it was positive although faint albeit positive.
13dpo was my longest - 8 dpo earliest
the day my period was due with my 1st i got a positive test
2 days before my period i got a positive test with my 2nd
tested the day period was due with my 3rd, got negative, tested a week later and got a positive test
this baby i tested 4 days before period and got a v v v faint positive on a test which was confirmed the day before my period with a digital test.
could it be you ovulated later than you thought?