My cat has a seroma ...

[Meesh]

Hi, Dr Lee

I've read a previous thread from a while back which related to a member's dog who had developed a seroma and was hoping you could give me some advice.
We adopted 2 cats from a shelter last year August - one of them, Trix, is about a year old now. After moving house in Jan, they spent quite a bit of time outside and often in the street. About a month ago, Trix started behaving very strangely and seemingly couldn't go to the toilet; we eventually noticed small amounts of blood around the house and he was in noticeable pain. He also had a deep wound on his front leg. After an overnight stay at a vet, which cost a small fortune, we were told that he may have been bumped by a car which would explain the trauma to his abdomen. He responded well to a drip and managed to go to the toilet several times thereafter and during his time there. He came home and took his medication nicely and looked much better. We thought he had made a full recovery until I noticed a sac-like, hardish "bulb" just underneath his tummy area. It resembled a hernia, but he wasn't in any noticeable discomfort. He received a scan and X-ray and we were then told that he had a seroma. The vet also showed us the fluid which was almost odourless and the colour of ginger ale. He couldn't, however, tell us what had caused it. He drained it and prescribed antibiotics; he also gave him an anti-inflammatory.
It is still there (a week later), albeit slightly smaller and I'm starting to feel increasingly more concerned. Even though the vet insisted that surgery wasn't really necessary, I don't know how serious this is and if it will go away on its own? PLEASE HELP!