Is there a chemical or herb in appetite suppressants that can cause you to grind your teeth constantly?

Possibly. Many appetite suppressants contain some sort of stimulant and this could lead to hyperactivity such as grinding.
Yes. I agree with dr harwood. However if grinding persists after discontinuing appetite suppressant, may want to have your occlusion checked by an orthodontist. Grinding can damage teeth, supporting bone/gums, and jaw joints. Ask your dentist about an occlusal orthotic to protect your teeth.