Unexplained implantation pains/twindges 6-10 dpo then 1days late AF?

This has happened twice to me now out of 4 months of TTC, Both times i had twinges 6 DPO then on and off twinges untill 10 DPO when i got very sharp twinges! It felt like implantation cramps, which i know the feeling because i already have a child) But dissapointly came on my period 1 days late :( Like i say this has only happened twice the other 2 months of trying not a twinge! Has anyone had a similar experience? and if it was implantation cramps what would be the reason as to why it didnt fully attatch and i got my AF and a BFN the morning my AF came ?? I would also like to say that since taking B vitamin suplements for 4 months my period's are usually like clockwork to the day.