Advice please?

So I'm 15 and have had pretty regular normal periods for 2 years (ish) but this year has been awful and very irregular. In April I missed my period completely, in may I had a period, 2 weeks later in June another period, 2 weeks later another period and now 2 weeks later another period! What is going on? But these periods have been very heavy often accompanied by cramps and terrible outbreaks. Is this just growing up or should I be concerned?