I have noticed that when I am confident I communicate in english reasonably well. Words and sentences come out very easily. But when I am not very confident or when I am not in a very good mood, I can't communicate very well, and I tend to make mistakes.
I don't know whether It happens only with me or it's natural. If it's natural does anybody knows whether it only happens to a newbie or to an expert of the language as well?
Very natural. When you are relaxed, you are less worried about making mistakes. Ironically (hmm... there's another thread somewhere about irony...) this means you make fewer mistakes -- or at least that if you do make mistakes, they don't make you worry.
Many people have told me they find it much easier to speak a foreign language after a glass or two of wine. This is because alcohol makes you confident. I don't recommend drinking more than that, though -- you might say something you'll regret...