What Causes Delays In Menstruation?

Hi, I came off mini pill in October. 5 months ago. I reverted back to a 4 week cycle, period every 4 tuesday following a day or 2 of aches and pains and that getting ready feeling. This week was expecting period last tuesday and got the usual aches pains etc Monday, Tuesday, Wednesday, Thursday but no bleed. Its now the following monday so 7 days later and still no bleed. Did a HPT yesterday which was negative. Im confused by all of this but also now concerned as my son has developed chicken pox today! What should I do?