my period came 3 weeks late but during that time my breasts was extremely sore. i havent done a pg test as yet. but my period is here now, so im wondering why is that??

Hello and welcome to EmpowHer,
There are many possible reasons to have irregular periods that are not related to pregnancy. In most cases, having a late period does not indicate any serious health issues. Most doctors recommend taking birth control pills it you feel it is absolutely necessary to have a regular period.
I suggest that you read about the possible causes at the link below.
https://www.empowher.com/menstrual-cycle/content/think-youre-pregnant-10-other-reasons-your-period-might-be-late
Faith