My brother's girlfriend was a care worker doing basically the same job and she was screwed over many times before. It's part of the reason why she quit in the end. However, it's a quality profession whose workers don't really get enough credit where it's due really.

Have you complained with a superior about the situation? It seems like a real cock-up on the part of your supervisor. However, it probably would be a good idea just to clear the air with the supervisor; don't want a negative atmosphere with your work colleagues.