Questionhi my friend is 17 and she haven;t got her period her three months but she never had sex the can be the problem.
Answer If she has not been menstrating very long. Meaning if she has only had her period for a couple years to begin with she may just be having a change in her cycle that should start up again.
If she has gained alot of weight or lost alot that could cause it.
Very active young ladies will also have this problem.
In any event she should mention it to her mom. Good luck to you.