Ah sis, that irritability after feeding at 11pm could be so many things, growth spurt, yes possible, pero pwede rin siyang uncomfortable pa lang sa digestion or just overtired na from the day! A few of us moms noticed our babies were extra fussy around that time kasi they're still adjusting sa sleep-wake cycle nila, especially gabing-gabi talaga. Since she's only 3 weeks pa lang, I'd really suggest checking with her pedia para ma-rule out kung may tummy issues or kung kailangan niya ng adjustment sa feeding, better safe than sorry with newborns talaga!