I think may something bother you which u cant share with any body.May be over mental stress,anxiety, harassment, any worse memory, any relationship problems, sudden bad news etc. U have to find the exact cause and then treated that cause with proper way. U may take external help to consult psychologist online or offline and learn how to deal worse conditions in life.

Its your thoughts. Thoughts can change your emotion in seconds. Whenever you experience that you are going towards negative emotion think of good memories which you had in your past. This might help you. Regards

Sometime it happens that some error components interefere with our thought process. That can change your state of mood. Thoughts regulate our emotions. So erroneous thoughts leads to negative mood state. Most of the time it happens without our conscious efforts. Some persinality issues are also found to be involved into this mechanism. I would suggest u to go for a detail evaluation from a clinical psychologist, if it is very distressing. Some psychotherapy sessions can help you to understand the problem and to learn how to cope with it.