What to do for sick 11 week old?

alt0925

I'm not sure exactly what she has but she feels warm, even though her rectal temp isn't high, she has flat out diarrhea, she's very fussy, vomiting with each feeding, not sleeping well, etc. I don't know what to do for her. On top of all of that, she has an open sore diaper rash. The pedi gave me sylvadine and ive been topping it with Vaseline so it stays in place but it's not getting better. It seems to be getting worse if anything. I have called the pedi about this, they just say to give her pedal it's and use sylvadine. I don't know how to help her. Any advice?