I get major downers, but is this normal?

Sometimes, and I admit it is more often when I am stressed, I get into a really down mood that lasts for several days. I can't concentrate, I can't get to sleep, but then don't want to get up in the morning. I end up doing little work and that makes me feel worse. And then randomly I will wake up one morning thinking everything is OK again. This I think is more or less normal.

What worries me is how often and how low I seem to get. about once a month or so, even with no real changes in stress levels, I get to the point that I just want to give up. This frequently results in me crying. And being me, I do this alone, and pretend there is nothing wrong afterwards. I'm Ok now, but I went through a phase where I would cry myself to sleep several times a week (but I am no longer in that situation), though it still happens on occasion.

I've always just let most of this pass, thinking I was just a wuss, or being overly dramatic. But the other day I was on one of these downers, and decided I wanted to bleed. Nothing major, I just wanted to draw blood. Having a low pain threshold, and almost not wanting to do it (but felt I really needed to) I ended up using the back of a badge/pin thing to scratch across my wrists. It wasn't anything big, only light bleeding, and welting, but it scared me that I got to the point that it felt like I needed to do that.

I do not think I am depressed, but I am not sure that my moods are entirely normal. I really just want advice. I went through a bed patch at highschool to the point that Mum and Pa were talking of getting me help (for depression). I said I was fine and they left it for a while and it didn't get worse, and then after I left school and went to college it got better. But yeah, I'm just not sure if I need help, or whether this is just me.

Thankyou so much