I don't think crying by itself is a sign of depression. I'm pretty driven & productive nowadays, and sometimes I still get inappropriate tears I can't explain. I just give myself a minute in the bathroom, let it out as much as I need to, then get on with my day. And if I have to head back to the bathroom for a few more minutes, I do it. I can't explain why it happens; sometimes I'm mystified.
There are times when I know it's hormones, I should mention. Or food. I'm female, and even at 28 years old there are times when I know it's an internal, chemical problem that's making me react differently to the world around me. But it took me a really long time to get that in-touch with myself. And anyway, it's not the only explanation.
If you have other symptoms of depression (for instance, long periods of listlessness/boredom), you may have depression.
I think it's important to take care of yourself, keep track of your eating & water intake (at least in a general way, like "I had breakfast today"), and try and get some activity in, even when you're just going through the motions.
I suffered from depression a few years ago (and previous to that in high school) and for me it was something I recovered from really slowly. But I think one positive thing I did (in high school) was that I still expended energy. Mind you, I didn't think I -had- energy at the time; hiking up to a lookout point near my house every day was more of a monotonous task than it was exercise (I didn't care what I was doing so it might as well be that). But I think when I was ready to release other issues, that's something that put me in the right physical state to do it.