First off I gotta say, good for you for actively potty training her at an early age!!! So many moms wait till their kids are 3-4 yrs old to potty train, for whatever reason, and that's their choice. But I found that potty training at an earlier age, with a willing and capable child of course, saved so much money from having to buy diapers.
I started potty training my daughter, passively, when she was 1, a little more aggressively when she was 18 mths. She was fully potty trained, both day and night by 21 mths. I had worked full time and she was in daycare all day.
For your situation, is the sitter maybe doing something different with her during the day? I agree there has got to be a reason she started refusing for you all of a sudden. It could be that maybe the sitter is doing something different with her, either giving her some reward you don't, or on the extreme side of things, maybe forcing her to sit for hours on the potty or reprimanding her for not going when she's on it. Not trying to say anything bad about the sitter or worry you, it's just difficult when your child is with someone else all day and too young to explain to you why she doesn't want to go. It could be something as simple as just giving mommy a hard time or not wanting to stop playing to go. It's hard to say. I would maybe back off just a little bit, like not putting her on the potty as many times as you normally would, but I wouldn't give up on it completely. A couple of minutes here and there is better than nothing at all. It may just be a phase which I think can be common when potty training at any age. Sometimes they want to go, sometimes they don't.
My daughter was very big on praise. Everytime she went I made such a big deal out of it, clapping, hugs and kisses, and tellng her I was so proud of her. She loved the attention.
Good Luck!!!