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Guinea Pigs should have fresh grass hay all they want, as well as 1/8 cup of Guinea pig pellets and a cup of fresh Veggies daily. They also require Vitamin C supplement daily beside what little is in the Pellets or Veggies. This is important. Guinea Pigs cannot produce Vitamin C in the body. Without a good diet, hay and Vitamin C supplement the are predisposed to Scurvy. This goes over the proper diet: http://www.guinealynx.info/diet.html
The Falling over on one side and the twitching is a sign of a Paraiste affecting the skin known as guinea pig mange Trixicara cavis. The seizure-like behavior is due to the intense itching sensation caused by the mites burrowing through the skin. With this being said I suggest that the Guinea pig see the Vet where they can treat for this problem. Make sure you are getting fluids and food going in to keep the Guinea pig from getting weak and have the Vet check it.