Seizure or heart problem?

My sister is younger and home alone until tomorrow. She called me freaking out about her cat, but I'm in another state and unable to help her so I'm hoping you can give me some insight. Morris was acting normal, crawling on her lap getting pet, when he suddenly jumped down and ran behind the couch and started howling as if in pain. She moved the couch to get to him and he ran into the kitchen where he laid down under the table. He was panting and drooling, and his eyes were black (sounds like pupils dilated). He lost control of his bowels and pooped all over himself and the floor. He laid there like that limp, panting, and drooling, for several minutes until he calmed down then laid there for about 5 minutes more. Then he got up, ate, drank, and was walking around again as if nothing ever happened. He is old (12), but has always been perfectly healthy. No issues, you would have no idea he was that old. He does go out in the yard occasionally (fenced yard, always supervised), but she said the last time was at least 3-4 days ago and wasn't for very long. We are both concerned for Morris, especially my sister who is now terrified he's going to die. I tried to do some research on the internet, but can't find anything of use. They can get him to a vet on Monday (no ER vets near them, they live out in the boonies). If he's acting OK now, is he OK? Is it poison? I thought seizures, but she said he was limp and laid still and that doesn't sound like a seizure to me so I am clueless. Any advice you can give would be very helpful. Thank you!

Thank you for your quick response! If it is a seizure, what kinds of things cause seizures and recover so quickly? Could it be just because he's old, or is it more likely he got hold of some kind of toxin...? And would pain be involved or could it just have been a howl of fear for the oncoming seizure?

Thank you for your quick response! If it is a seizure, what kinds of things cause seizures and recover so quickly? Could it be just because he's old, or is it more likely he got hold of some kind of toxin...? And would pain be involved or could it just have been a howl of fear for the oncoming seizure?