Growling, meowing and panting?? I'm scared!

Long story short, i adopted a feral recently. He was TNRed a month ago, has been on treatments of fendendazole, metrodanizole, and now the last course of fenben since yesterday because he has had persistent diarrhea. He is FIV pos. and 4 years old.
I had a screen door installed so he could stay in, away from my cat. Last night was the first night he stayed in and did not make a peep.
He has been eating for two earlier in the week (while on metro), but had not been eating the past 3 days. I get him to take his fenben with tuna juice, and has eaten 1\3 of a can of A\d about 3 hours ago right fater his fenben. He has been sleeping in the cat tree since then (4 hours). 
I was just in the kitchen (where his base camp is) to do dishes and put clothes in the dryer when i heard him growling from up on the tree. He meowed, so i shut the dryer off in case he was frightened.
I resumed the dishes but he kept meowing, ending those meows with panting. Panting? He did that 3 times and settled after a few minutes.
What's that about? Why do resting cats pant?
He is now down the tree and meowing at the door to be let out of the kitchen.
Should i be worried? Heck, i already am.
He is being seen by a vet on thursday, under sedation, cause he still has the runs (at least i witnessed he did 2 days ago),. I am planning a complete blood work. 

I went in the kitchen to find clay paw prints in my table. He had gone to pee in the box, but did not bury it. He's never done that.

I should mention i have never seen him strain to pee, and he produces normal pees. He only peed once last night, so he doesn't exhibit the drinking lots\peeing lots thing associated with UTI.

Panting and growling for apparent reason is not normal for cats, it is a sign of extreme pain and stress. I suggest you do NOT wait and take him to the veterinarian.

He asked for the door and i let him out. He had just eaten an ok amount of A\d and i reckoned i didn't want to make things worse by keeping him inside by force.
He is there this morning, and he ate the food i left out overnight. He seems ok now, for some reason. Wanted to come in, was telling me about his night, etc.
Maybe it really was just him being terrified of the dryer. But i think it's strange he didn't come down from the perch to hide somwhere or ask for the door. He just layed there, meowing and panting.

He is eating right now, on his own.