Hi thanks all for your answers.
I brought her to the vet last friday and her molars are all ok. She had her upper teeth clipped to an appropriate length and weighs about 550g, she put on 100g since december.
We dont know why she has no lower tooth as the rescuer found her that way.
The vet told me to reduce her pellets so that shed eat her hay. I did that and she is beginning to touch her hay, although not much. But will continue to monitor.
Also she is on oxbow pellets so i think she should be fine.
So for now, is it better to bring her to have her teeth clipped every month?