5 months doesnt drink milk

Hi everyone. My baby has never had any problem drinking milk. Just recently when she turned 5 months, she suddenly doesnt want to finish up her milk. Usually she will finish up 120ml at one feed but now most of the time she will drink until 60ml and sometimes 40ml only. We try to give her interval every 30 mins, hoping she will drink bit by bit but it doesnt work, she still doesnt want to drink. We try to wait 3/4 hrs for her next feeding hoping she will feel hungry and finish up her milk. Still she will only drink until 60ml only. Is it a phase? Im worried she might not be gaining weight much and PD will consider her as unhealthy. Last check her weight was 6.2kg