Advise please

Shravana • Blessed with baby girl 11/1/17 😊
Hi ladies,
My last cycle - May 20 - CD 1
Started Clomid 100 mg - May 22 - CD 3
Positive Ovulation test - June 8 -  CD 20
Day 21 Progesterone blood test - June 9
As per my blood test results on CD 21 the value was 4.04, which shows I haven't ovulated till that day but my OPK was positive just the day before so possible I might have ovulated after my blood test FX. 
Today, I am 15 DPO. I started my periods but only see little amount of blood when I wipe and also noticed a streak of brown-colored blood. No heavy flow at all - my pad is all white but I see blood only when I wipe. 
I took a HPT yesterday which was negative. 
Now, my question is 
Would this be implantation bleeding and is it possible that I got a BFN yesterday as my levels might be low ?
Is it that once you have implantation bleeding, for sure you get a BFP