This happened to me with my first.
Turned out (and this is gross) the sitter wasn't refrigerating the bottles. I transported them in this little freezer pack to the sitter's, and she wasn't putting in fridge. Alternatively, she just sometimes left one out on the counter, and thought that was ok. When I approached her about it (she was sending them back) she said she thought they were formula (?) and ok to leave out.
This was a lady with 20+ years experience. We just weren't on the same page. I didn't think I'd have to specify, but glad I went through it thoroughly with her when I just couldn't get what was happening.
I have had friends say that sitter's didn't heat it up to same temp (different warming system) or not at all ...
Or maybe she doesn't get rid of the air in the bottle - some babies can be fussy.
Maybe ask your sitter to show you how she feeds her when you are still there - just go a bit early and don't feed her a full bottle before you head over. That would be my suggestion. If baby won't take it if you're there (want you to give it to her) you could just make yourself scarce so she doesn't see you and see how it goes. I agree - I'd be concerned about her cutting back that quickly.
One other thought - do you have some routine where she takes bottles fro you? Like do you rock her or sit with her before nap or anything? Because sitters will (understandably) have to change things up from routines you had in place. It takes a while for little ones to adjust. Could that be it?
Good luck I hope you get it figured out :)
ETA: If you've been breastfeeding and she's not used to bottles - I would try different bottles. I had to go through 3 types of nipples (right flow, the ones that felt most like a breast, etc.) before we got one they liked. Playtex I believe is what we ended up with in the end with the disposable liners.