Beginner Gardening: Lemon tree in pot on terrace starting to show yellow leaves , 4 by FR06

FR06 wrote:
Lemon tree in pot on terrace starting to show yellow leaves and some of the very small fruit are turning brown
Have ordered a soil moisture meter and given the plant "mediterranean fruits" plant food bought last year - it says it contains Potassium Oxide and Phosphorus pentoxide
I have had the tree 3 years. Last year no flowers at all, this year flowers and all going well till 2 days ago - I attach pictures - leaves have turned yellowish on one branch, and the fruit has gone brown and died on the branch

I also have a Mandarin and Kumquat but still these will not flower

I have been worried about over or under watering so now ordered a meter but at a loss to know what the problem is as first time growing citrus

Thanks