Does somebody know the reason for this brown leaf?

I have a citrus tree (buddhas hand) which grew very good this year. It has lots of fruits. Unfortunately the leaves began to have a small brown spot at their tip. It was raining then for about two weeks straight and was getting pretty cold. So i moved the plant into its winter residence (heated glass house). Due to the brown spots i fertilized with blue corn (nitrophoska) which has lots of potassium and nitrogen in it. This enabled me to fertilize without adding more water. I have also fertilized once with a special iron fertilizer (3%iron but thinned according to manual).But besides that not added any water. This was about two weeks ago. The door of the house was opened except two very cold nights. But it also was raining the whole time,so high humidity. The soil is still a little humid. To the cold nights: i have started the heating system but i cannot exactly regulate the temperarure(i can regulate the gas flow,but do not know which temperature this yields to). The min temperature was about 4°C. Today i have noticed that 4leaves are getting brown and 2of them are laying on the floor. Could someone help me and tell me what the reason is or what i could do about it?thank you very much :)