Sunken fontanelle at 5 mos. Is it normal?

Kelly • First baby on the way after 4 yrs of trying 😍

Hello. My lo is 5 months and i noticed she has sunken fontanelle. It doesn't stay long though sometimes it is there then suddenly it's gone. She is happy and active no other issues. I haven't noticed this before with her. She feeds every 4 hrs with 4oz breastmilk sometimes sleeps through the night with one feed only and sometimes full night without feed from 8pm to 5am. Should i be worried or this is normal?