Let me put your mind at ease.
If there is no way that sperm can have gotten inside your vagina then there is no way that you can be pregnant.
It can take several years before your periods settle into a regular cycle.
If you say you miss a month then get it the next maybe your cycle is just like 40 or something days long.
Count the days from the first day you start bleeding for your period right through until the next time you have the first day of your period.
Knowing this will help you know when to expect your period.
Incidentally this can change as you get older.
I have had a regular as clockwork cycle of 26 days since I was 9 years old and suddenly for no apparent reason for the past 3 cycles I now have a 29 day cycle.