Sorry I can't help with your problem, but I'd think it unlikely that the drugs are to blame. My Izzy has been on increasing amounts of Vetmedin, Fortekor (another heart med) and Frusicare ( a 'water tablet') for over two years, since he went to grade 6, with no such problems, and your vet would be informed of possible side effects.
I hope your dog is well again, soon and that the seizures were just temporary. My trainer's sheltie had three or four fits (one absolutely horrific), then no more. The cause was never found.
Barbara, Monty, Joly and Teddy.