12 years of AF! 2 months of weridness!

AF has alway been normal and on time. I always had pms sysmptom the week before and Af wpuld show up right when the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> would date it as. I have had my period since i was 9 year old. I am 21 year old now so i literally know and feel when it coming to the point it like second nature like breathing. Recently my last two period have been coming late and my pms symptom are completely no existence except for a a week before and the week of af, i am completely nauseated and have heart burn . Is this normal? I have never had a change in 12 year , why would this happen.