Often until a female has a child her period doesn't regulate. I've had mine for like 3 years and mine is still very irregular. I asked my doctor and she said it's normal to have irregularity in periods. Don't fret, I once had one 2 weeks after another and then a month and a half in between, it's normal.
#1school; or #2your bloody stuff #1 if school you have problems #2 you may have to see the doctor and if you reaallly dont want to wait anthor 28 days and if you miss it again you might change your scheadule if your more active and runing excissise witch mixes everything up you have new times