3m+ baby seems to be on milk strike, increasingly fussy

Starting this week my 3m+ old baby started to drink much lesser in 24h.. there is no clear sign of what caused this change. She was sick (nasal congestion/cough) previously but have since recovered and milk intake regain back to normal level before this.. Now she drinks half of what she used to drink. she cries loudly even when placed in the position ready for feeding. At times she may just take 30ml and reject further milk by crying.. we’ve tried both breast milk and formula milk, same reaction. Feeding interval became v long too almost 4-5hours nv cry for milk. She also developed new habits eg only likes to be carried upright. So far no fever, still peeing ok. What should we do? When will it become a concern when we should go PD for a check? #FTM