Srdjan Pamtivek:
Is this something usual? How to overcome it? What happened?
Not unusual at all.
First thing you need to do is to is reduce physical agitation as much as possible. In the suttas, this is called "calming bodily fabrication", but all it means in practice is that you still and relax your body as much as possible. Really settle down. Go through each part of your body, systematically releasing tension. A good way to think about this is that you want to avoid pushing or pulling the breath from anywhere in your body. Even if you can't get the mind to settle down, still devote yourself fully to calming the body, systematically, piece by piece. The mind can only be so agitated when the body is very calm, so even if you're not having a perfectly still mind, it's still a bearable experience, and you're likely to come back to do it at the appointed time tomorrow (which you should always do).
Now, I've come across some good techniques for getting the mind settled on the breath, but there's no guarantee they'll work for you or that they'll work every day. So greet it with a spirit of exploration.
One idea is to do what Daniel Ingram calls "grooving on the breath", where you get into the rhythm of the breath the way you'd get into a good piece of music.
Another approach, which Daniel also advocates, is to imagine you're savoring the smell of something really wonderful, like a gardenia, a lilac, something like that. Really be fully present at the tip of the nose, as if every second were the only second you had the chance to smell this wonderful fragrance.
You might also try taking refuge in the breath. This one works well for me. It can be wonderfully liberating to be focused so closely on the breath, because there's no room to think about unpleasant things. To test it, think of something unpleasant right now for ten seconds, and then focus exclusively on the sensation of the breath at the tip of the nose for just three inhalations/exhalations. Very nice, right? The more you get into the breath like that, the less susceptible you are to unpleasant thoughts and feelings (hindrances), the better you feel. This is called "happiness arising from seclusion". That's good motivation to stay with the breath. Treat it like a vacation.
Good luck, and let us know how it goes.