Why am I having reduction in breast milk?

What is the age of the baby? If it is 5 months, the reduction could be because of decreased feeding of baby, pregnancy, increased activity on your part with decreased calorie intake. These are the commonest causes. Eat more, rest more, feed your baby more(even during the night), and from 6 months onwards, start top feeds with soft, semi solid fresh home made foods. By 1 year the baby must be taking in half of what you eat in a day.