In that case, you do nothing.
Your keep a written record of your monthly cycles for the next 2 or 3 months and if they are still not regular, you make an appointment to see your doctor to check your iron levels and your hormone levels.
Sometimes, the period can fluctuate and come a few days earlier or a few days later and that is quite normal.
So don't worry about it. That's nature for you.
I never had sex before so there's no chance of pregnancy,but I don't know what's happening to me.
Periods can come a day or two early or late.
If you have had unprotected sex, then there is a risk of pregnancy.
Depending on how many days/weeks late you are, you can purchase a home pregnancy testing kit. Some pharmacists do pregnancy testing and your doctor can arrange for it to be done too.
There is not much else you can do apart from wait to see what happens or get a pregnancy test done.