no period for 7 months

I am 15 years old and have had my period since grade 6. it used to come regularly of about 3-4 days every other month. about 7 months ago was my most recent period. i havent had it since. my mother keeps telling me not to worry , but i cant help it. have you ever experianced this? If so what did you do, and what was the issue if there even was a problem.
please, put me at ease lol