Weight problems

Hi mommies, before I got pregnant petite talaga ang katawan ko as in payat ako. I weighed only 45-48kgs. Ngayon, I got pregnant with my first baby and it seems parang wala masyadong nadadagdag na weight sa'kin even though malakas ako kumain. Ang liit pa ng tiyan ko for a 5-month pregnant and I only weighed 51kgs as of my last check up. Idk po if normal yun pero naaalarma na ako. :-(