Baby struggling to feed at 11 weeks

Hi all, our 11 week baby is suddenly struggling and pulling away from the bottle, we haven’t had this issue in quite awhile. His past feeds have usually been quite smooth except when he was colicky, furthermore he’s taking almost twice as long to finish his feed now. He’s feeding about every 3-4 hours so it should be normal. We did recently change to the non-HA version of Nan milk, replacing about 3 feeds in a day to ease him into it. Could that be the reason? Any advice or experience would be appreciated!