I have really bad periods

Okay so I'm 22, and not Prego. My periods are all confusing sometimes it takes 3 weeks before my next period or 4 weeks and some times 5 weeks before I start....I've been eating fast food a lot simply because I work all day and get home to late to even cook...can my diet be the problem or can it be something else...? (Oh, yeah I also get yeast infections)