Phalaenopsis base is turning yellow

[meliessa]

Hi everyone
i brought this this phal a few months ago, after is flowers died off, it has starting to lose leaves, now the whole base is yellow and only one leave left but the roots still looks healthy to me. It is planted in a finish bark mixture and i don't water it too often i give water maybe once a week or next depending if it looks dry to me. is it some fungal infection or did i over water it and killed it?

thanks!