What Causes Irregular Periods?

hello, i used to have a very regular period at every 27 days, but for 3 months now is coming to early around 21-22 days and is starting first day with a brown colur not red like before. it's not painfull but i noticed it takes longer;it was 5 days but now can take up to 7 days of period.i'm not on birth control, i'm 30 years old and 2 children 5 and 2 years old.it's someting abnormal now with my period, thanks!!!