Worried about my baby weight

Hi , my daughter is 4 months & she looks skinny I can see her ribs when she lays down & I can feel them when I’m holding her. she is formula feed, sometimes when I feed her she arch her back or she stop during for a bit then come back for more. It’s like she doesn’t really drink a full bottle at once. She is a long baby , at her 2 month check up she was 12 pounds almost 13 I don’t know if it’s due to her stretching but I feel like she losing weight and I am so worried 😢, I’m a first time mom and I just really need some advice. The last picture she was 3 months. How do you know if your baby is getting enough milk ? Should I add rice cereal for weight gain? Please help

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