Frozen Breast milk refusal

Any ideas on what to do if baby won't eat frozen BM? I have been pumping since birth but supply is pretty non existent due to several factors. I had a lot saved from the beginning and today he's refusing and crying at the bottles I have been giving of thawed milk. I don't have any more fresh it is fortified with high calorie formula so don't have to go straight to that? Of course it's the weekend and I can't call my doctor