Get your diet sorted out… that’s one reason it happens to me . It’s not uncommon at all for this to happen during your late teens and early twenties. It’s been a re-occurring theme in my life from time to time as it has for many people. If it gets bad go get help but otherwise just learn to roll with it and do your best to try to identify and rectify the things that are getting you down. It’s not always a bad thing, it can be your minds way of getting your attention that something is not right.