Diarrhea ( watery poo ) is not normal in a healthy ball python and could be the result of a parasitic infection. This may or may not be the cause of her refusal to feed, but it is something that may have to be addressed with a vet visit.

If she is starting to get thin, then you could try assist feeding, but avoid full on force feeding as it is usually counter productive.

Was the Ball Python thin when you recieved it?

Watery stools and loss of body fat are not things that usually occur over a period of 12 to 13 days IMO.