My 3.5 months old girl drinks very little milk 3- 4 oz every 3 hrs and 24 oz in a day. The guideline on the milk states that by now she must be drinking 6 oz per feeding. I have tried changing teats, formula milk brand, different positions of feeding but still it's same. She dozes off after drinking 2 out of 4 oz and will just sleep until next feeding. Is this normal? Her weight is in the 10th percentile. Petite girl.