Baby is extra fussy after drinking breastmilk

Hello everyone! I’m a new mom and my baby is exactly 1 week and 2 days old today. Her latch was perfect from the get go and for the first 3 days of life I exclusively breastfed, but noticed that she would not stop crying and would only sleep for about 15-30 minutes at a time, then feed for nearly 30 mins per breast! Naturally, me and my poor breasts suffered so I had get an OK from my doctor to formula feed her. Now she sleeps like an angel and has wakeful moments without crying since I tried mixed feeding, but I noticed she is extra gassy and fussy after I breastfeed her. Maybe my milk doesn’t sit well with her tummy? Has anyone encountered this?