Cluster feeding and formula??

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Hey mommas! My little one started cluster feeding yesterday afternoon. No matter how long he feeds, it seems like he only finally calms down and gets full after i give him a bottle of formula (or breast milk if I happen to have any pumped). I only do this after I finally get to the point where my nipples hurt and just need a break. Is this normal? Or is this a sign he simply isn’t getting enough from me? Any ideas??