True seizures are rare in guinea pigs.  If your guinea pig cries out, falls to one side, and starts twitching, it is most likely infected with a skin parasite known as guinea pig mange (Trixacarus caviae).  The seizure-like behavior is due to the intense itching sensation caused by the mites burrowing through the skin.  Some guinea pigs may have mites and a normal coat of hair.  Other guinea pigs will develop patches of hair loss.  In the span of a few weeks, a guinea pig may become bald over most of its body. Read here for more information on guinea pig skin parasites.

In Arizona, true seizures are frequently caused by a guinea pig getting too hot and developing brain damage.  Other common causes are ear infection, a head injury (getting dropped onto the floor),  infections, tumors, and poisons.  Many guinea pigs may never have a cause found and this is termed "idiopathic epilepsy".  If your guinea pig has seizured, do not delay getting to a veterinarian!  An exam including bloodwork and other diagnostic tests should be done as soon as possible.  If you suspect your guinea pig got too hot, this is a life-threatening emergency! Cool its feet with rubbing alcohol mixed with water and place it on a damp washcloth until you are able to get to the veterinarian.