If your Lionhead rabbit shakes or trembles but otherwise seems active and alert, it is probably frightened by something. If your Lionhead shakes or trembles and is lying down, it is probably seriously ill. Make sure it is in a space with good ventilation and a comfortable temperature (around 68° F/20° C is ideal). If the problem is not excessive heat or cold, then it may be food poisoning, parasites, or infection. Isolate the rabbit so it does spread any infections, and take it to the vet for treatment.