My kitten isnt walkin on his hind leg?

I dont know if this relates to this but my family has been using hartz deworming meds on my kitten for 3 days while ive been away at my friends. While playing with the other kitten, out of no where he started to complain and my dads gf started holding him and 3-5 minutes later he started shaking, about 30 seconds in between each one. Now he has been favoring one of his back legs and he tried to walk on it but meowed when he did so. He hasnt tried walking on it since. She touched his leg to try and figure out where it hurt but he didnt meow at all. Why is this happening?