Implantation bleeding?

Could use some advice please... I tend to go by the glow app to track ovulation, which has been pretty accurate as to when AF is due. According to Glow I was due AF yest 5 Jul. Last Thurs 1st (10dpo) I had about 2 hrs of spotting, then Friday I had one wipe where I had a tiny bit of stringy pink blood, no blood since. My breasts became very sore after this happened & still sore today (6th), with lower back pain. I tested today & got a BFN. I've never had spotting before but think it could be implantation bleeding? When should I retest?