I have been angry recently and I don't know why. I sometimes just want to break down and cry. My friends don't know of my feelings because I hide it from them. I don't want them to feel down. I just don't know what to do.

Shout, it works, al least for me ^^

Find some form of exercise you enjoy. Aerobics, yoga, tai chi, weightlifting, boxing, and so on. Don't ignore your stress, work with and around it. Try thinking about what bothers you and take certain measures to alleviate those problems. Do something that benefits you.

Just tell them, express yourself. Just like Jackal said, do something you enjoy and it'll relieve your stress and you'll feel a lot better :)

ignoring your feeling is bad. and that is going to eat you up from inside if you
don´t do anything about it. i suggest talking to your friends. because friends are supposed to be there for you. i bet you would like to know if any of your friends felt this way as well? and i bet you would help them out as well. i know that they will be there for you and help you out ^_^because thats what friends are for ;)

You appear to be upset due to keeping your emotional stability to a minimum. Subconsciously, though you feel as though you'd be burdening your friends, it's like even if you did, they wouldn't very well know how to deal with it. What I'm getting at here is that perhaps you're not angry because you're hiding your emotions, but rather because it's so easy. So why hide it at all? Let it out and tell them how you feel. Ultimately, this is the best path to take in this scenario and will either broaden your bond's horizons, or heighten your standards. Either way, this could only serve as an experience to make you better for it.