I didn't read the other responses, so forgive me if anyone else has said this:
DD1 was always like this...almost didn't even make it onto the growth charts some months.
For a while the pedi even had us adding an extra scoop of formula to her bottles for the extra calories. (so I would put 3 scoops into either 4 or 5 oz of water)
Has your pedi suggested anything like that?
With DD, it even continued as she ate more solids, went off formula, etc. We saw a nutritionist, had every blood test imaginable, etc.
Some kids are just small/pettite. I know it must kill you that CPS was called, but if it's standard protocol, try not to be offended by it. You voiced your concerns several times. It's clearly nothing you did wrong.
(Now that DD1's older we can give her Carnation Instant Breakfast or Pediasure, which seems to help her put on weight)