It could be a lot of things. Here are a list of drugs and toxins that can cause seizures.
More than likely it was some sort of poison as that was her area of expertise. Most any substances taken orally will have a delayed effect. In this case, I don't believe it was too long after.
She most likely took too much knowing it would cause a seizure but also probably knew that they'd take her to a hospital before it killed her. At that point though I don't think she really cared if she lived or died as long as she didn't have to go to jail. At least that's what the visiting scene with Arlene made it seem like.