Extremely concerned

My baby girl is going to be 12months old in November, but I am extremely concerned about her development. Though she is perfect on other milestones like activities and crawling-cruising etc, her height and weight are not very good.

She is only 6.5kg and looks like a 6 month old baby. She easily fits into 3-6 month baby’s cloth. She showed 6(4 upper and 2 lower) teeth in one month and she has diarrhea most of the times. She also has a slight squint in her left eye.

What freaks me most is that she seems to be moving all the time. She does the crawling position and moves forward and backward as if doing some kind of exercise. Even if she is sitting she does a forward and backward movement as if she is riding. Is it normal? Any help please (she was born 2.4kg and 43 cm long).