Rhianon, if you have a 28 day cycle and it is mostly regular, it'd probably be pretty safe to assume that you'd not fall pg on day 6, I would've thought. There's a bit that goes on in the pre-ovulation phase - a dominant follicle needs to have time to develop to a certain level of maturity in this time, before it is able to release the ovum. Also, in the unlikely event that fertilisation would have occurred, the endometrium needs to have time to repair itself after menstruation for implantation.
When I was charting, I always considered the few days after my period 'safe'.
HTH a little.