missed pill & late period

I recently went on holiday & missed 3 pills whilst away & me & my partner had numerous unprotected sexual activity. I carried on taking the pill as normal once I realised I'd missed the others & still have been taking them normally up until the day of my period usually starts & have not taken one since. Last year I had one injection of the depo injection but never progressed with this form of contraception as it made me emotionally unstable so I started the cerazette pill around June & still been getting regular periods on the same day but I am now 5 days late, feeling abit nauseous as soon as I wake up but not being physically sick, back pain & stomach cramps are also happening frequently. What do you think is happening?