toddler started regressing

Hello Dr,

My 17 month old daughter has started to regress. She could say a few words like bye bye, baba, dada. Wave her hand when saying bye bye. Listen when her name was called. However she has just recently in the last month or 2, stopped saying these words and doesnt wave bye bye. Often doesnt look when we call her by her name but will listen if we call her by something she likes. Her eye contact has lessened as well. She can walk and is playful, likes affection, pretend plays for example feeding a doll. Pretends to comb hair when given a comb. Since she was small she was underweight and in the last 3 months she has gained 1 kilo with the help of the supplement pediasure. She is 17 months and 9.3 kilograms. She also got sick when she was smaller however her development was good and we didnt have any problems until she started regressing a month or 2 ago. Please advise us on what we can do and what medicine we can give.