I have 2 daughters, both were born 8 days past their EDD, both were 8lbs1oz at birth. my older daughter then skyrocketed and was 75-90 %ile for a while before settling eventually to about 50% for height and 25% for weight (she's 6). Child #2 has steadily fallen on the charts and now at 13 months is only 16 lbs and 28 inches. Her head circumference has stayed at 50%ile and her thyroid was tested, but her doctor is not terribly worried since DH and I are both on the thinner side (I'm 5'3" and maybe 112, He's probably 5'11" and 160). However, DH was a big baby like our older daughter and I think I was more average (my mom says I doubled my birth weight7lbs8oz, by 6 months, which would put me much bigger than this baby).
She's extremely active and happy. She eats everything and is ahead in all her milestones (crawled at 6 months, first steps at 8.5 months, walking at 10 months). Her poops aren't runny, but occasionally are more frequent which gives her diaper rash (which usually clears up pretty quickly). She also gets restless in the morning for the last couple hours of her sleep which may or may not be related to teething. Her skin looks good, she isn't overly fussy or sick, just tiny and active.
Should I be looking into food intolerances? I did try a gluten-free dairy-free diet for a few weeks when she was an infant because she would refuse to nurse and would scream when I tried too nurse her from time to time, but it didn't seem to help and once I added gluten back in, she seemed to actually be better.