Start recording the starting date of your periods and look for a pattern - count the days between the starting dates of your periods. Either use your diary or try one of the free phone apps for period tracking. Periods are often irregular for the first year or so, then they settle into a predictable routine of arriving every 28 days or so. Every body is a little different so there can be some variation in this and for some girls, irregular is normal. Having sex in itself does not alter when you should expect your period, unless you became pregnant, then you would expect no period until after the pregnancy. To know one way or the other, buy a pregnancy test kit form the supermarket or pharmacy or see your GP. Always use protection during sex to protect from pregnancy and STIs.