I agree with yinyang on the personal stuff, but just wanted to add something else. I was exactly like you when I was around 14 or so (not sure of your age, but same symptoms anyway) and I had nothing happening to cause it. It's only now, looking back, that I realise it was probably puberty and hormones and all that stuff going on that just upsets your normal bodily balance. It can go on for ages while you're growing up (no offence meant!) and everything is still changing. There's such a lot happening over that time that takes years to adjust to. I was crying at school on and off for many months - eventually it all subsided, but it still makes me feel sad to think about it, and to think that it's happening to someone else.
My advice would be to make sure your parents are aware, or a teacher, even if you don't necessarily feel you want to say it out loud. My mum didn't know how I felt and I always wish I'd told her. But the best thing is to spend lots of time with your friends, don't wait for one of them to organise a get-together - you be the one who suggests a dvd night, a trip to the cinema, a long knackering walk somewhere, clubbing, festivals, camping?? Anything! You sound young - too young to be sad for no reason, and it breaks my heart. Don't let it get hold of you. Acknowledge the fact that it happens, but don't try to understand it as it may get a better grip. Try not to worry too much, and think of that feeling as an irritating person trying to provoke you, then don't give them the chance. Keep yourself busy, and remember that it will pass.
Also, last thing, if you do feel the need to talk to your GP, don't put it off - it's all too easy to feel you don't need/deserve/want to see one!!
Lots of love sweetheart, chin up and take care.