Vet is definitely the best option. Pet stores are horrible places to look for advice. The employees don't have to have any sort of qualifications or knowledge about caring for the animals. You're better off just asking on internet forums.
But what you're describing sounds kind of odd... Were there any cuts or anything on him? Is the vent swollen? If there was a regurge at any point, I'd expect to see other stomach contents floating around...

You need to post the answers to the Trouble in the Enclosure questions so people can help figure out what went wrong if you can't get a vet's opinion.
http://www.frogforum.net/pacman-frog...enclosure.html

Also, what do you mean when you say you have him isolated now? Was he housed with other animals before this? If so, that may be the cause of whatever injury/illness you're looking at.