Is this normal and what do I do

Before I start I just want to say that I am a 14 year old girl that has never had sex. So my periods started when I was 12 I had about 17 days in between them. Once I turned 13 a year ago they stared to get really heavy and come ever 15 days. Now they are really heavy and come ever 2 weeks (14 days) this app has said my cycle length is only 21 days is this normal and how do I fix it?