My cow decreasing her milk from 15 litre milk per day to 7 liter milk per day

Subclinical mastitis is the first thing I would suspect. A sudden drop from fifteen liters to seven liters is not normal. First check the udder for heat, pain, swelling, or clots in milk. Milk each quarter separately and do a CMT test if you can. Keep the cow on clean dry bedding, wash the udder before milking, and milk her gently and fully. Give good green fodder, enough concentrate, clean water, and a mineral mixture. If there is fever, hard udder, foul milk, or she is off feed, she needs a vet today. Treating this now is far cheaper than losing milk for the rest of the lactation.