Get really sad

I know! That is not really good when you have no reason to be sad and still you are. I feel like I am doing things no one needs at all. Plus I get really nervous about plenty of things. Sometimes I know what is the real cause of those worriations but today I have been trying myself to the edge, checking what is going on with me constantly and found that I get nervous 'cause of nothing. I am sad but no one or nothing caused it. Plus I feel real tension in my head