Fluid in abdomen

My kitten, Merlin, is about 7 1/2 months, and was found in a parking lot as a tiny kitten.  He was dewormed, but at one point I found vomit with living roundworms, so he was dewormed again twice, 3 weeks apart.  Last week I called the vet because his belly was still swollen, sides bulged out, so we gave him Profender.  When I had another cat at the clinic today, my vet asked how Merlin was doing, and I told her his belly was still swollen.  She asked me if it felt like a water balloon, and I said yes.  She said she wants to do an ultrasound, which is scheduled for 3:00 this afternoon.

In the meantime, I've been reading online about stomach fluid, and the prognosis is grim.  He is healthy, growing like a weed, shiny fur, very active, eats great, no vomiting- no signs of anything amiss except his belly and he has pretty chronic diarrhea.

Has anyone else out there been through this and came out okay?  I'm so scared at what it could be.

I would not do too much reading at this point and let the ultrasound and the vet do the diagnosis - you will only fret yourself into a panic.  It may be nothing, or it may be serious, as you have read.  At this point, it's the Serenity Prayer.  I will also say a prayer for a positive outcome.