Growth from 2.6 kg to 5.2 kg by 3 months is good progress, so this isn't a growth concern. Around this age babies become much more aware of their surroundings. Getting distracted during feeds and taking in less milk at a time is a very common phase, and it often comes with fussier sleep too.
Next Steps
Try feeding her in a quiet room with fewer distractions, and offer the bottle when she's calm rather than overtired. Short breaks to burp during the feed, known as paced feeding, often help a distracted baby take in more.
Health Tips
Please see your pediatrician if her weight gain slows, she has fewer than six wet diapers a day, or feeding keeps getting harder over the coming week.