Do not feel guilty. Honestly if he had not been sedated they may have not been able to find the mass, especially if he was wiggly or aggressive. The mass may have also been painful to manipulate. Sometimes it is better to knock them out completely and do a thorough examination. The gas anesthesia is a little quicker to recover from as opposed to injection, and there is less risk of getting scratched or bitten by a frightened feline. Some cats wake up quickly and are perfectly fine, while others take it a little rougher. It is difficult to predict how any given cat will react, but it usually makes them a little nauseous afterward. he was probably sick from the anesthesia but sometimes stress or fear adds to the adrenaline. My old cat used to vomit and poop everywhere and then hide after sedation even though I hadn't fed him since the night before.

Please do not beat yourself up over this. 16 years is a good long life for a cat. Some don't make it that near that long. You realized he had a problem and once you discovered what it was, you got to enjoy him a full 7 months longer. Now that he is gone, the what-if's and woulda-coulda-shoulda's will drive you mad, but you have to let them go, as there are never any satisfactory answers. Just rest in the fact that he is no longer suffering and that you got to enjoy his company and love as long as you did.

RIP TommyCat's kitty