She may be going through a growth spurt. Is it cluster feeding ie does she feed less at other times of the day? There is no need to top up with fm if her diaper output is good and weight gain is good. It's tiring to keep feeding, but persevere, mummy!!
Might be she's not drinking enough. Have you tried pumping and bottle feed to see how much baby drinks and how's your supply?
Should be your milk supply is not enough for her. It doesn't matter how long you bf. Its the amount you produce. You might want to top up with 20ml fm first while you boost your supply? A usual supply bfing session would take amount 30 min.
pap_1