What Causes Irregular Periods?

Hi I have a ? The past two months my periods have been late they usually come on the 20th every month and its came on the 29th and 30th and I dont no whats really wrong I took a pregnancy test cause I thought I could have been pregnant cause my periods only lasted about three days.and were unusawally lighter and there always pretty heavey and this time around I noticed.when I went to wipe after using the bathroom that there was a chunky brown looking cecreation and Ive never had this happen to.me ..... Any suggestions ? Or answers I just dont think.its normal