Irregular?

I've been on the pill for about four years now, started being super conscientious about taking it properly when I started having sex about ten months ago. For the most part, my cycle has been regular, I almost always get my period on a Monday or a Tuesday, it's now Friday and it hasn't come yet. I work with seven other women aged 19-30 and almost all of them have had issues with their cycles being irregular this month. Has anyone noticed their cycles changing to match the others around them when they're on the pill?