Is the cat still eating/drinking and keeping the food/water down? Able to go potty normally? Playing, energetic, acting normal? If no on any of these, then DEFINATELY see a vet ASAP.

If cat is eating, drinking, playing, not tender in that area; then I would call the vet and ask for advice on what else to watch for or their recommendations on what you might be able to do for your kitty. Sometimes things don't show problems for a day or two and your kitty may get very sick by that time. Calling your vet should help ease your mind, or at least give you more specific information on what you should do.

Good luck.