Breastmilk that doesn’t have enough nutrition.

I have just been told that my breastmilk might not contain enough nutrition for my baby for my baby’s weight has not been improving for almost a week. His current weight now is 2.63kg, the past few days his weight is stuck between 2.63 - 2.67 kg. The doctors has told me that my milk is too watery, not enough fat in it. May I know what has caused this? And how can I improve my breastmilk so that my little one can get enough nutrients that he needs. I’ve turned to family and friends and they suggested me to give formula instead of breastmilk, for I know it is also good, I would very much prefer to give my baby breastmilk for I know it’s the best milk for them.