Decreasing light levels at this time of year could be responsible, or is it in a draft. It may be worth trying to see if it resonds to a different location with a bit more light. Also remember not to over water in the winter.
yip,it`s in a draught,i thought that it would be ok as it`s an outdoor plant suitable for indoor growing.It has lots of light,i`ll try positioning somewhere else.thank you