Anyone????

Period has been coming twice a month. It came right on time on the 26th of may like my app told me, lasted until june 2nd. It was due today, june 18th and i haven't got it. I have been having lower back pain though i took a pregnancy test this evening and it was negative. I know its ok to be about a week late, but i am not stressed out at all. Im just wondering what could be going on? I am also 3 months pp 2nd child, formula feeding i got my first period may 3rd babygirl was born march 8th.