Refusing to breastfeed

My 4 month old little girl has started to refuse breastfeeding during the day (we bottle feed at night) She's always taken to breast and fed for 10-30mins depending on the time of day. The past few weeks she's just screamed and cried with nipple in mouth (I use a shield), I can see there's a milk flow but she's just not having it! I'm worried this isn't a phase and will slowly turn into complete refusal and I rarely have time to express so I'm worried my supply will drip! Help! Support or advice would be amazing!