I am so sorry!! The truth is, it could be a lot of different things. She may need to taper the Prednisone more slowly. She may be dehydrated, the 6mp can cause nausea and vomiting if the dose is too high. Or it might just be a badly timed virus. I would give her GI or pediatrician a call and get some zofran (anti nausea med) and have her sip gatorade. If she is having severe abdominal pain, it could be an obstruction. That would be unlikely considering she just finished Prednisone. But that would mean a trip to the ER. I also would call the GI first thing and check in with them.
Try not to panic, I know easier said than done. I have had many days I thought everything was falling apart and my son managed to get through it, it was just a bump in the road.
I will pray for you and your daughter this morning. We are here for you, let us know how Ashley is doing. (((((Hugs))))))