Refusing BM bottles, taking formula

Cheerio B

DS has been refusing his BM bottles at daycare for a few weeks now. It takes them an hour to get just 4 oz in him. He's nursing OK at home, not great. I can tell based on his diapers that he's getting dehydrated. Out of desperation we tried a bottle of formula at DC and he sucked it down. Same thing the next day, 30 minutes to get 1.5 oz of BM in him. He sucked the formula bottle straight down.

Does anyone have any ideas on what could be causing this? I've offered both fresh and frozen milk. My diet hasn't changed. I'm not on any meds. I've worked so hard to BF him and this makes me so sad and frustrated.  He's gone through a bottle strike before but he nursed better at home through that so we weren't worried about getting fluids into him.