Thanks for all the "great and usefull" advices.  
So I did some googling and found the best way how to solve this. If anyone interested, search "The Sims 3 Neighborhood Workshop".

Well, that's not the answer I was actually looking for I mean is it ok that this is happening after changing her weight and going back to the game? Or i just do something wrong...

Using "editsim" I make my sim very slim, then go back to the game and after few second her body weight changes to default. How can I keep her slim? I have also Jonha Sliders from MTS installed..