Hi,
It's possible that the human shampoo did irritate his skin in this tender area and he's having a reaction.
If the actual skin of his anus is red and swollen, it's also possible that he has an anal sac problem and they have to be expressed by the vet or treated with antibiotics if they're infected.
http://www.sniksnak.com/cathealth/analsac.html
The best thing you can do for Tiger is bring him to the vet tom'w, to find out for sure what's causing this, and then have it treated.
If he's not interested in eating or drinking, you have to get some nutrition and fluids into him. Feed him by dropper, one dropperful of a water mixture (or pediatric unflavored Pedialyte, if you can get it) of 1 cup water with a pinch of salt and sugar, every 10 minutes for an hour. Then mix some canned cat food with the Pedialyte or water mixture to make it soupy enough to get into the dropper and give him one dropperful every 20 minutes for an hour. Alternate the water and food for 4 hours or until he starts eating on his own. (If he should vomit, stop!) This should temporarily prevent him from getting dehydrated or damaging his liver from not eating/drinking.
Is it possible that during the bath with human shampoo, that he ingested some of it with the lather/water, and that's also what's making him feel ill? Was he rinsed well? Is there a residue on his coat which he licked during grooming?
Definitely get him to the vet tom'w, first thing, or, if you feel it's necessary, bring him to a vet ER center near you, tonight.
Please keep me posted on how he's doing and what the diagnosis is. Thanks!
Cher
If my answer is helpful please click ACCEPT. Click reply for further clarification. My only goal is to provide 100% helpful answers. Thanks for the opportunity to help!