TTC - when does implantation bleeding actually happen?

Hello ladies, looking for some advice....

According to glow I am 12 days past ovulation, should have been due lady time yesterday, but nothing. Today I have had some light spotting when wiping twice, which now seems to have gone. Did an early test 5 days before due on which was negative (probably too early to test, just couldn't wait!). Could this be implantation? Does it happen at this point?

Husband and I are having a bit of a rocky ttc road 2 miscarriages :( and 2 more very early suspected :(. I have pcos, so we always knew things could be difficult.

Feeling desperate....

Advice would be greatly appreciated xxx