Weight gain is very critical in initial years of life especially first 3 months of life for growth and development of child. First 7 or 10 days, a child may lose 10% of body weight but then should start gaining weight. Good you want to only breast feed the child but then we have to monitor the child whether he is on the right track. Urine frequency should be 6 plus in 24 hours each day and child should gain weight of 150 to 200gm per week. If this is not happening then Breastfeeding is not enough for the child. Get checked for breast feeding techniques, position and breast milk output so to find out the issue and get it solved. Don't wait for something to happen on its own. We have to be proactive in this phase.