Weird period cycles? Help please!

Hi, I'm an 18 year old and I've been having extremely strange periods this year. Jan-March I was regular, all year before that too! (10th-15th each month. Always on time.) and then all the sudden I didn't get my period till July 20th! It was really heavy (4-6 super tampons a day) and lasted about 8 days. And now, (August 28th) I have not gotten my period at all. Now, here's the thing. I have NOT had unprotected sex between  March and July. I haven't had any sex since my last period. Is anyone else having weird periods like this? Should I be more worried than I am? Is this normal for someone my age? Please help. Thank you.