It's just changing of time.its normal.but it will be good if u do pregnant test.but if u found u r negative then u r in normal condition. Dont worry.
Your monthly cycle can change over time. If you are not sexually active it probably is nothing to be majorly concerned about. As J10PL said stress weight changes, vitamins can affect your cycle. Even a change of friends or environment can affect your cycle.
My little girl and me usually have ours at the same time, but if I dont see her for a month or so mine will change a bit. She has said the same thing. So it is normal, but honestly talking to your mom wont hurt. If you feel comfortable doing so, then it is good to have those talks with her and that way she can help ease your mind.
Lots of things can affect your cycle - vitamins, weight changes and especially stress. You could go to your doctor or a women's health clinic or speak to your mum. Try not to worry too much and get checked out by a doctor.