OK so recently I began playing Skyrim again and got the Dragonbron DLC set up.

Everything looks and works fine but there is one problem with my beast mode... every time I turn into a wereworlf after 5-4 seconds I transform into a human... I never had that before the DLC

I couldnt find anything about it on the net, I found(on another site) some guy having the same problem but he was modding and seems that was the cause

I dont have any mods on my game ? is that some bug or I did something that effect my beast mode in the game story ?

Pleas help !

That is odd. Is this a save from before you got the DLC set up or a new game? The DLC might have somehow glitched your Beast Form, so starting a new game may help if you have not already.

Also, might I suggest you try the Unofficial Patches for Skyrim and Dragonborn? I highly recommend them for everyone as they help with many bugs.

Skyrim-> http://skyrim.nexusmods.com/mods/19/?

Dragonborn-> http://skyrim.nexusmods.com/mods/31083/?

If the problem still persists after you have tried my suggestions, you might want to try a mod which changes the length with which you can remain in Beast Form such as Werewolf Mastery.

It seems you lack Hircine's favor.