What made my 12 year old yorkie who was fine all day, tail wagging, eating and seemingly normal suddenly start

to pant and then collapse and have like a seizure. Her body just laid while she shook and could not move for about an hour and a half? SHe ate, peed and went back to lay down afterwards this happened last year about this time also. I took her to the vet and they didn't find anything wrong except she does have a oversized heart. I am limited on money with a family of four so I didn't take to vet to be told they don't know whats wrong with her again. Does anyone out there know or had this experience with there dog?