Are you sure it's the hernia? Incision sites often swell slightly due to a reaction to the suture. Give it about two weeks and if it hasn't gone down then I'd say it could be the hernia. Any pus or blood coming from the incision?
I'd urge you to bring the cat back immediately so that the shelter staff/Vet can examine the cat - people on the forum really cannot tell from your description alone exactly what may/may not be going on. I would NOT suggest a "wait and see" approach.