I wouldn't replace the bf with a formula feed if she's frustrated (the ones you are usually offering, not the daycare one). If she's unwell, I'd suggest you offer her a few more frequent bfs, or maybe even try bfing her when she's sleepy, relaxed, in the bath, darkened room, etc. Have a look at the suggestions for breast refusal and perhaps some of those will work.

I hope she's feeling better soon. I agree with the others that 8 mths is too young to be weaning, it sounds like breast refusal.