Skipped period

Okay so I'm 5 days late with my period, I am bang on 26 day cycle and we had sex using the pull out method (yes I know this is not full proof) on the day before my most likely ovulation day, and the day after. I have set pms symptoms every month. Been off contraception now for 7 months. Now my thing is I have had no pms symptoms, and also no period. I did a pregnancy test first thing this morning and it was negative. I don't have any symptoms except being exhausted (fell asleep at 7.30pm one night) not even sore boobs (though sometimes tingles nipples)
My previous pregnancies I've no idea when I may have found out I was pregnant in the cycle of things due to never ever keeping track of my cycle.
So if I'm not pregnant what other things could cause a skipped period and is it likely I just haven't waited long enough to have a positive reading yet?